154. In speaking of inordinate exercise before meals, I have alluded principally to bodily exercise; but let me here observe, that the undue exertion of the mind is equally hostile to the due performance of the digestive functions; and when we consider the vast importance of nervous influence in carrying on their operations, we cannot feel surprised should a person, after he has been jaded by mental anxiety, or exhausted by intellectual exercise, be incapable of digesting an accustomed meal. These observations suggest many important questions for the deliberations of those who are placed in superintendence of public and private seminaries; the school-cry of "Duty before pleasure," has scattered the seeds of disease, even amongst the most robust of our species.

155. I have already explained the necessity of exercise at that period of the digestive process, when the chyle enters the circulation (123); and it is, perhaps, not the least of the evils which attend the modern fashion of late dinners, that it should preclude the possibility of such a regulation. The utility of dancing may certainly be deduced from these views, and its propriety sanctioned on just principles; but the lateness 1 of the hour at which these recreations commence, and, what is worse, the excessive heat and ill-ventilation of the apartments in which they are usually carried on, must counteract any benefit which might otherwise attend an indulgence in them. If exercise be useful during the period of sanguification, pure air is no less so; and I shall take this opportunity of entering my protest against the introduction of gas into the interior of our houses. Carburetted hydrogen is a deadly poison; and even in a state of great dilution, it is capable of exerting a very baneful effect upon the nervous system. I have been consulted on several occasions for pains in the head, nausea, and distressing languor, which evidently had been produced by the persons inhaling the unburnt gas in the boxes of our theatres.

In order to afford additional support to the objections which 1 have urged upon this occasion, I shall quote an account of the effects produced upon Sir Humphry Davy, by the inspiration of carburetted hydrogen gas. He introduced into a silk bag four quarts of this gas nearly pure, which had been carefully produced, from the decomposition of water by charcoal, an hour before the experiment, and which had a very strong and disagreeable smell. "After a forced exhaustion of my lungs," says he, "the nose being accurately closed, I made three inspirations and expirations of the gas. The first inspiration produced a sort of numbness and loss of feeling in the chest and about the pectoral muscles. After the second inspiration, I lost all power of perceiving external things, and had no distinct sensation, except a terrible oppression on the chest. During the third expiration this feeling disappeared, I seemed sinking into annihilation, and had just power enough to drop.the mouthpiece from my unclosed lips. A short interval must have elapsed, during which I respired common air, before the objects about me were distinguishable. On recollecting myself, I faintly articulated, ' I do not think I shall die." Putting my finger on the wrist I found my pulse threadlike, and beating with excessive quickness.

In less than a minute I was able to walk; and the painful oppression on the chest directed me to the open air. After making a few steps, which carried me to the garden, my head became giddy, my knees trembled, and I had just sufficient voluntary power to throw myself on the grass. Here the painfid feeling of the chest increased with such violence as to threaten suffocation. At this moment, I asked for some nitrous oxide1. Mr. Dwyer brought me a mixture of oxygen and nitrous oxide, which I breathed for a minute, and believed myself relieved. In five minutes the painful feelings began gradually to diminish. In an hour they had nearly disappeared, and I felt only excessive weakness and a slight swimming of the head. My voice was very feeble and indistinct: this was at two o'clock in the afternoon. I afterwards walked slowly for about half an hour, and on my return was so much stronger and better, as to believe that the effects of the gas had disappeared, though my pulse was 120, and very feeble. I continued without pain for nearly three quarters of an hour, when the giddiness returned with such violence as to oblige me to lie on the bed; it was accompanied with nausea, loss of memory, and deficient sensation.

In about an hour and a half the giddiness went off, and was succeeded by an excruciating pain in the forehead, and between the eyes, with transient pains in the chest and extremities. Towards night these affections gradually diminished; at ten no disagreeable feeling except weakness remained. I slept sound; and awoke in the morning very feeble and very hungry. I have," adds Sir H. Davy, "been minute in the account of this experiment; because it proves, that carburetted hydrogen acts as a sedative, i. e. that it produces diminution of vital action, and debility without previously exciting. There is every reason to believe, that if I had taken four or five inspirations, instead of three, they would have destroyed life immediately, without producing any painful sensation 1." After this proof of the poisonous nature of c:ir-buretted hydrogen, - after the cases of sickness and headache which have occurred, in consequence of its inhalation at the theatre, am I not borne out in my opinion, that its introduction into our apartments is objectionable2?

1 See Plutarch's Life of Cicero.

1 In former times the ball commenced at six, and terminated as eleven; but now it begins at eleven and ends at six.

1 Sir H. Davy had previously inspired this gas, and found it capable of producing an excitement resembling that of incipient intoxication.

156. We may, in this place, very properly take into consideration the evil effects produced by the inspiration of impure air. It may be stated, as a general proposition, that all living bodies when crowded together generate a matter which would seem to be highly destructive. No species of animal can congregate in ill ventilated apartments with impunity. Under such circumstances, the horse becomes infected with glanders, fowls with the pip or pep, and sheep with a disease peculiar to them, if they be too closely folded3. Our chemical tests for the detection of animal impurities in the atmosphere, have hitherto been inadequate, and we have therefore been erroneously led to question the presence and extent of such contamination; and the palpable unhealthiness of crowded assemblies has been referred rather to temperature, or to the diminished proportion of oxygen in the air, than to the true source of the evil. We may, I think, look forward with some confidence to an improved system of eudiometry; and when this shall have been established, I will venture to predict that many important facts connected with the public health, and with the origin and propagation of epidemic diseases, will be disclosed.

It has been lately discovered that certain substances, in the state of minute division, have the power of absorbing animal matter from the atmosphere, and of rendering its presence sensible by yielding ammonia 1, on exposure to heat; such are pure sand, lime,gypsum, etc.; to this absorbing faculty may, perhaps, be attributed the advantages derived from whitewashing infected apartments, and from strewing the floors with fresh sand.

1 "Researches, Chemical and Philosophical, chiefly concerning Nitrous Oxide, and its Respiration, by Humphry Davy".

2 This objection of course points to the escape of unburnt gas; if the combustion is perfect, watery vapour and carbonic acid are the only products, with, perhaps, a small proportion of sulphurous acid. The heat generated by the combustion of gas when compared with that produced by oil lamps is as 3 to 2.

3 It is worthy of remark that these diseases, evidently engendered by congregation, become subsequently contagious. In the expedition to Quiberon in 1795, several transports, crowded with horses, had their hatches shut for a considerable time in a storm, by which some of them were suffocated, and amongst the surviving horses the contagious disease called glanders was propagated. At another period it was proposed to send live stock from England across the Atlantic, but the animals all died of a febrile disease in a few weeks, in consequence of being too much crowded.

1 This fact may be illustrated by the following simple experiments. Heat some common sand red hot, in order to expel from it every trace of animal matter; when cool, introduce a portion into a small glass tube, in which, a piece of test-paper has been placed. - Heat the sand by means of a spirit lamp, and the test-paper will not be found to indicate the presence of any ammonia. Repeat this experiment, with another portion of sand which has been touched by the hand, and the test-paper will how show the development of the alkali. In like manner, quick-lime, or gypsum, after exposure to an atmosphere impregnated with animal matter, will betray the contamination by a similar indication. It is, perhaps, worthy of notice, that oxide of iron possesses the same imbibing property; and the discovery of the fact is connected, in a very interesting manner, with a trial for murder which lately occurred in Paris.

157. The observations which have been offered upon the congregation of animal bodies will equally apply to the vegetable kingdom; indeed Nature would seem to have established the law, in order that the extent of her productions might be limited to those bounds which are essential to the well-being of the whole - but let us return from this digression.

158. Sleeping after dinner is a practice of very questionable propriety; it is true, that the inhabitants of many southern climates indulge it with impunity: but it does not appear essential in our country, where animal food is used in such considerable quantities. Its effect is to hurry on the latter stages of digestion, and hence the fever and state of excitement in which the person not unfrequently awakes. In states of disease it may occasionally be useful, and the recumbent dagger had been found, with spots of rust, in the apartment of a suspected person. The chemist was called upon to ascertain, if possible, whether these spots had been occasioned by blood. He accordingly examined them by the aid of heat, and finding that ammonia had been given off during the process, he concluded that they must have originated from animal matter, and delivered his opinion accordingly; but Vauquelin, one of the most celebrated and acute chemists in France, submitted to a similar examination a sword that had contracted rust in the ordinary manner, and he was surprised to obtain the same production of ammonia.

The fact, although at the time incapable of explanation, disarmed the evidence of its force, and the prisoner was acquitted posture may expedite the passage of the aliment out of the stomach into the intestines; but the person who lies down for the accomplishment of such an object should be careful to remove all ligatures from his body.