Lexicon medicum, or, Medical dictionary : containing an explanation of the terms in anatomy, botany, chemistry, materia medica, midwifery, mineralogy, pharmacy, physiology, practice of physic, surgery, and the various branches of natural philosophy connected with medicine : selected, arranged, and compiled from the best authors / by Robert Hooper.

Robert Hooper

Date:: 1832

Credit: Lexicon medicum, or, Medical dictionary : containing an explanation of the terms in anatomy, botany, chemistry, materia medica, midwifery, mineralogy, pharmacy, physiology, practice of physic, surgery, and the various branches of natural philosophy connected with medicine : selected, arranged, and compiled from the best authors / by Robert Hooper. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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ATM augment in bulk when they receive moisture, and diminish again when they part with it. Hence Borne of them have been employed as hygrometers, or mea- sures of the quantity of moisture contained In the air around them. This they do by means of the increase or diminution of their length, occasioned by the addi- tion or abstraction of moisture. This change of length is precisely marked by means of an index. The most ingenious and accurate hygrometers are those of Saus- sure and Deluc. In the first, the substance employed to mark the moisture is a human hair, which by its contractions and dilatations is made to turn round an index. In the second, instead of a hair, a very fine thin slip of whalebone is employed. The scale is divided into 101P. The beginning of the scale indi- cates extreme dryness, the end of it indicates extreme moisture. It is graduated by placing it first in air made as dry as possible by means of salts, and after- ward in air saturated with moisture. This gives the extremes of the scale, and the interval between them is divided into 100 equal parts. The water, which constitutes a component part of the atmosphere, appears to be in the state of vapour, and chemically combined with air in the same manner as one gas is combined with another. As the quantity of the water contained in the atmosphere varies con- siderably, it is impossible to ascertain its amount with any degree of accuracy. i 3. Carbonic acid gas.—The existence of carbonic gas as a constituent part of the atmosphere, was ob- served by Dr. Black Immediately after he had ascer- tained the nature of that peculiar fluid. If we expose a pure alkali or alkaline earth to the atmosphere, it is gradually converted into a carbonate by the absorption of carbonic acid gas. This fact, which bad been long known, rendered the inference that carbonic acid gas existed in the atmosphere unavoidable, as soon as the difference between a pure alkali and its carbonate had been ascertained to depend upon that acid. Not only alkalies and alkaline earths absorb carbonic acid when exposed to the air, but several of the metallic ozydes also. Carbonic acid gas not only forms a constituent part of the atmosphere near the surface of the earth, but at the greatest heights which the industry of man has been able to penetrate. Saussure found it at the top of Mount Blanc, the highest point of the old continent; a point covered with eternal snow, and not exposed in 11 to influence of vegetables or animals. Lime-water, diluted with its own weight of distilled water, formed a pellicle on its surface after an hour and three-quarters exposure to the open air on that mountain ; and slips Of paper moistened with pure potash, acquired the property of effervescing with acids after being exposed an hour and a half in the same place. This was at a height no less than ]5,6G8 feet above ifte level of the sea. Humboldt has more lately ascertained the exist- ence of this gas in air, brought by Mr. Garnerin from a height not less than 4280 feet above the surface of the earth, to which height he had risen in an air-bal- loon. This fact is a sufficient proof that the presence of carbonic acid in air does not depend upon the vici- nity of the earth. Now, as carbonic acid gas is considerably heavier than air, it could not rise to great heights in the atmo- sphere unless it entered into combination with the air. We are warranted, therefore, to conclude, that car- bonic acid is not merely mechanically mixed, but that it is chemically combined with the other constituent parts of the atmosphere. It is to the afhmly which exists between carbonic acid and air that we are to ascribe the rapidity with which it disperses Hself through the atmosphere, notwithstanding its great specific gravity. Fontana mixed 20,000 cubic inches of carbonic acid gas with the air of a close room, and yet half an hour alter he could not discover the traces of carbonic acid in that air. Water impregnated with carbonic acid, when exposed to the air, very soon loses the whole of the combined gas. And when a phial full of carbonic acid gas is left uncorked, the gas, as Bergman first ascertained, very soon disappears, and the phial is found filled with common air. The difficulty of separating this gas from air, has hitherto prevented the possibility of determining with accuracy the relative quantity of it in a given bulk of air but from llir experiments Which have been made, we may conclude with some degree, of confidence, that ATM it is not very different from 0.01. From the experi- nnint8S'f Humboldt>jt appears to vary from 0.005 to km ^his variation wi hv n<> means appear impro- bable, if we consider that immense quantities of car- bonic acid gas must be constantly mixing with the atmosphere, as it is formed by the respiration of ani- mals, by combustion, and several other processes which are going on continually. The quantity, indeed, which is daily formed by these processes is so great, that at first sight it appears astonishing that it does not increase rapidly. The consequence of such an increase would be fatal, as air containing 0.1 of carbonic acid extinguishes light, and is destructive to animals. But there is reason to conclude, that this gas is decomposed by vegetables as rapidly as it forms. 4. Bodies found in the atmosphere.—From what has been advanced, it appears that the atmosphere con- sists chiefly of three distinct elastic fluids united together by chemical affinity; namely, air, vapour, and carbonic acid gas; differing in their proportions at different times and in different places; the average proportion of each is, 98.6 air 1.0 carbonic acid 0.4 water 100.0 But besides these bodies, which may be considered as the constituent parts of the atmosphere, the existence of several other bodies has been suspected in it. Itisnot meant in this place to include among those bodies electric matter, or the substance of clouds and fogs, and those other bodies which are considered as the active agents in the phenomena of meteorology, but merely those foreign bodies which have been occa- sionally found or suspected in air. Concerning these bodies, however, very little satisfactory is known at present, .as we are not in the possession of instruments sufficiently delicate to ascertain their presence. We can indeed detect several of ihem actually mixing with air, but what becomes of them afterward we are unable to say. 1. Hydrogen gas is said to have been found in air situated near the crater of volcanoes, and it is very possible thatit may exist always inavery small propor tion in the atmosphere , but this cannot be ascertained till some method of detecting the presence of hydrogen combined with a great proportion of air be discovered. 2. (Jarlmretted hydrogen gas is often emitted by marshes in considerable quantities during hot weather. But its presence lias never been detected in air; so that in all probability it is again decomposed by some unknown process. 3. Oxygen gas is emitted abundantly by plants during the day. There is some reason to conclude that this is in consequence of the property which plants have of absorbing and decomposing carbonic acid gas. Now as this carbonic acid gas is formed at the expense of the oxygen of the atmosphere, as this oxygen is again restored to the air by the decomposition of the acid, and as the nature of atmospheric air remains unaltered, it is clear that there must be an equilibrium between these two processes; that is to say, all the carbonic acid formed by combustion must be again decomposed, and all the oxygen abstracted must be again restored. The oxygen gas which is thus continually returning to the air, by combining with it, makes iu component parts always to continue in the same ratio. 4. The smoke and other bodies which are continually carried into the air by evaporation, &c. are probably soon deputed again, and cannot therefore be con- sidered with propriety as forming part of the atmo- sphere. 5. There is another set of bodies, which are occa- sionally combined with air, and which, on account ol the powerful action which they produce on the human body, have attracted a great deal of attention. 1 tiese are known by the name of contagions. That there is a difference between the atmosphere in different places, as far as respects its effects upon the human body, has been considered as an established noint in all age's. Hence some places have been cele- brated as healthy, and others avoided as pernicious, to the human constitution. It is well known that in pits and mines the air is mien in such a state as to suffo- cate almost instantaneously those who attempt to 103i

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