Although these considerations were sufficient to make it desirable that someone should take up an investigation of such general utility, yet the difficulty of the researches which it involved would have prevented me giving myself to it, if I had not been strongly urged to undertake it by Citizen Berthollet, of whom I have the honor to be a pupil. I owe to him the apparatus needed to carry out this work, in which I have often been guided by his advice and that of Citizen Laplace: such great authorities will increase the confidence with which it may be received.
The researches which I have undertaken on the law of expansion of gases and vapors, and on the progress of the thermometer, are not yet complete. In this memoir my object is only to examine the expansion of gases and vapors for a fixed rise of temperature, and to show that it is the same for all these fluids; but before giving an account of my experiments I think it will be well to give a historical sketch of what has been done in this field; and as I shall introduce at the same time some observations on the different methods which have been employed, I will before entering on the history mention one of the principal causes of uncertainty which can arise in this sort of experiment. Although it is very important and although it seems to have been unrecognized by most of the physicists who have studied the expansion of gases, it will be sufficient for me to mention it to make clear what its influence will be.
He had also sought to determine the expansion of water-soluble gas, and he had found for each a particular dilation different from that of other gases
This cause of uncertainty is the presence of water in the apparatus. Suppose, in fact, that some drops of water are left in a globe full of air, of which the temperature is raised to that of boiling water; this water will turn into vapor and will occupy a volume about 1800 times greater than its original volume, and thus will then necessarily happen that when these vapors are condensed so that they occupy a volume 1800 times smaller, there will be attributed to the air which remains in the globe much too great an expansion; because it will be assumed that it is this air which, at the temperature of boiling water, occupied the whole volume of the globe. If the temperature is not carried to this degree the same cause of uncertainty will nevertheless exist, and its importance will depend upon the temperature at which the experiment is tried: for in this case the water will not be entirely vaporized, but the air will dissolve more and more of it as its temperature rises, and will receive in consequence a greater and greater increase in volume besides that which it gets from the heat; so that when we go again into the lower temperature, the volume of air which fills be2Â login the globe will diminish for two reasons, 1. because of the loss of caloric, 2. because of the loss of the water which it holds in solution. We shall then still attribute to the air too great an expansion.
In general whenever we enclose with the gas liquids or even solids, for example muriate of ammonia, which can dissolve or vaporize at the temperature to which they are exposed, there necessarily result errors in the determination of the expansion of these gases
Before going further, I must jump ahead. Although I had recognized on many occasions that the gases oxygen, nitrogen, hydrogen, carbonic acid, and atmospheric air all expand identically from 0° to 80°, citizen Charles had noticed the same property in these gases 15 years ago; however, since he never published his results, it is only by great luck that I knew it. In this respect, my experiments differ strongly from his.
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