A Lecture on the Preservation of Health | Page 4

to affect him so much, as to deprive him of sight altogether.
Let us next consider what happens with respect to heat; if heat be for some time abstracted, the excitability accumulates; or in other words, if the body be for some time exposed to cold, it is more liable to be affected by heat, afterwards applied; of this also you may be convinced by an easy experiment--put one of your hands into cold water, and then put both into water which is considerably warm; the hand which has been in cold water, will feel much warmer than the other. If you handle some snow with one hand, while you keep the other in your bosom, that it may be of the same heat as the body, and then bring both within the same distance of the fire, the heat will affect the cold hand infinitely more than the warm one. This is a circumstance of the utmost importance, and ought always to be carefully attended to. When a person has been exposed to a severe degree of cold for some time, he ought to be cautious how he comes near a fire, for his excitability will be so much accumulated, that the heat will act violently; often producing a great degree of inflammation, and even sometimes mortification. We may by the way observe, that this is a very common cause of chilblains, and other inflammations. When the hands, or any other parts of the body have been exposed to violent cold, they ought first to be put into cold water, or even rubbed with the snow, and exposed to warmth in the gentlest manner possible.
Exactly the same takes place with respect to food, if a person have for some time been deprived of food, or have taken it in small quantity, whether it be meat or drink; or if he have taken it of a less stimulating quality, he will find, that when he returns to his ordinary mode of living, it will have more effect upon him than before he lived abstemiously.
Persons who have been shut up in a coal-work from the falling in of the pit, and have had nothing to eat for two or three days, have been as much intoxicated by a bason of broth, as a person in common circumstances with two or three bottles of wine; and we all know that spirituous, or vinous liquors affect the head more in the morning, than after dinner.
This circumstance was particularly evident among the poor sailors who were in the boat with Captain Bligh after the mutiny. The captain was sent by government to convey some plants of the bread-fruit tree from Otaheite, to the West-Indies; soon after he left Otaheite, the crew mutinied, and put the captain and most of the officers, with some of the men, on board the ship's boat, with a very short allowance of provisions, and particularly of liquors, for they had only six quarts of rum, and six bottles of wine, for nineteen people, who were driven by storms about the south-sea, exposed to wet and cold all the time, for nearly a month; each man was allowed only a tea-spoon full of rum a-day, but this tea-spoon full refreshed the poor men, benumbed as they were with cold, and faint with hunger, more than twenty times the quantity would have done those who were warm, and well fed; and had it not been for the spirit having such power to act upon men, in their condition, they never could have outlived the hardships they experienced. All these facts, and many others which might be brought, establish beyond a doubt the truth of the law I have mentioned, namely, that when the powerful action of the exciting powers ceases for some time, the excitability accumulates, or becomes more capable of receiving their actions.
The second law is, that when the exciting powers have acted with violence, or for a considerable time, the excitability becomes exhausted, or less fit to be acted on, and this we shall be able to prove by a similar induction. Let us take the effects of light upon the eye; when it has acted violently for some time upon the optic nerve, it diminishes the excitability of that nerve, and renders it incapable of being affected by a quantity of light that would at other times affect it. When you have been walking out in the snow, if you come into your room, you will scarcely be able to see any thing for some minutes. Look stedfastly at a candle for a minute or two, and you will with difficulty discern the letters of a book, which you were before reading distinctly; and if you happen to cast your eyes upon the sun, you will not see any