June 27, 1890] 



SCIENCE. 



387 



ever, may be partially avoided by conducting operations at 

 variable rates of speed, because, if we obtain the same re- 

 sult with different rates, we can conclude that the lagging is 

 inappreciable. Again: the amount of this lagging may be 

 approximately computed by using two thermometers with 

 different rates of lagging. Whatever may be this lagging, 

 however, it is plain that it would be exactly the same in 

 moist as in dry air, so that our comparative results will be 

 entirely free from any error due to the instrument. In 

 saturated air, moisture collects on the bulb; but this can 

 make no difference, since in saturated air the dry and wet 

 thermometers read alike. 



Experiments. 



On rapidly compressing the air and suddenly releasing it, 

 the amount of rise and fall in temperature was the same, 

 about 7° for 10 inches of compression. We shall be entirely 

 within the limits of error if we assume the lagging to be 1°. 

 Since the air at the side of the jar was at very nearly the air 

 temperature, we can consider that the average heating and 

 cooling of the whole air was not far from 4°. This shows what 

 an enormous loss of heat was sustained by the air in com- 

 pression to one-third its bulk. The theoretical heating 

 should have been 163°, while the actual heating was one- 

 fortieth of that. But the most important fact in this con- 

 nection is that Espy, under the same conditions, found the 

 cooling after expansion to be nine times that found above. 

 Espy emphasizes the great necessity that exists in closing the 

 stop-cock at the moment the mercury reaches a level, or at 

 the exact moment when we may suppose the cooling by ex- 

 pansion is greatest. On repeating these experiments, it was 

 a matter of great astonishment to find that the delay of a 

 few seconds only in stopping the expansion, after the col- 

 umns in the gauge were at the same level, almost entirely 

 obliterated the subsequent rise. This would seem to show 

 that heat from the environment had little or nothing to do 

 with this rise, and this is an exact corroboration of the in- 

 dication of the thermometer; for a rise of 4°, which was found, 

 would represent less than .25 of an inch on the gauge. Does 

 the mercury in the gauge reach a level before the air inside 

 the jar is in equilibrium with the outside air? Professor 

 Marvin has suggested that the momentum of the mercury in 

 the latter part of the expansion would cause it to reach its 

 level sooner than the air its equilibrium. On performing 

 the experiment at different speeds of expansion, it was found 

 that a definite relation existed between the rapidity of fall 

 of the mercury and the subsequent rise after arresting the 

 expansion. For example: the air each time was compressed 

 to 400 millimetres, and expanded in 5, 10, and 20 seconds. 

 The amount of rise in these cases was approximately 41, 21, 

 and 12 millimetres respectively. It might be thought that 

 this was due to the greater accession of heat to the air during 

 the slower expansion (that is, the cooling would not become 

 as great), but in all these cases the thermometer indicated the 

 ■same cot^ling at the end. It must also be plain that this 

 effectually disposes of the question of lagging, as suggested 

 above. The evidence is cumulative and conclusive, that the 

 rise noted by Espy was not due to a heating, from outside, of 

 the air cooled by expansion; and his whole theory regard- 

 ing the difference in cooling, of dry and moist air, falls to 

 the ground. 



As regards the fact that there is only an exceedingly slight 

 rise after waiting a few seconds. Professor Seaman has sug- 

 gested that the air may be heated in these few seconds, and 

 therefore there is no rise. But it takes time to heat the air 

 under these conditions, and in these few seconds the amount 

 of heating is exceedingly slight: in fact, this one thing is a 

 strong argument against the view that the rise noted after 

 expansion is due to heat; for the rise is vei-y rapid, and is 

 accomplished in a few seconds. Still another point is to be 

 noted : by making the compression to 400 millimetres very 

 rapid (that is, in 10 seconds), and then expanding in 5, 10, 

 or 20 seconds, we have heated and cooled our air by very 

 nearly the same amount, and it has come back to just a de- 

 gree or two below the outside air temperature; so that the 

 subsequent rise cannot be due to the accession of heat from 

 outside. There is an exceedingly interesting matter right 

 here that I leave for physicists to consider. We are told 

 that the only way in which air can lose its heat is by per- 

 forming work. Now, is it possible for us to consider that 

 the amount of work done in compressing the air to 400miDi- 

 metres is exactly counterbalanced by the work done by the 

 air in forcing aside the outside air as it rushes out of the jar? 

 It would seem as though the former must be a thousand or 

 more times greater than the latter, if we take account of all 

 the circumstances. One more experiment was tried to de- 

 termine the cause of the rise in the gauge after explosion. 

 If this rise were due to the behavior of the gauge, rather 

 than to outside heat, we ought to be able to obtain it at any 

 moment after expansion, and long before any marked cool- 

 ing had taken place. When the expansion was arrested 

 after one or two seconds, there was a marked rise in the 

 gauge. This arrest must have been long before the cooling 

 could possibly have brought the air to the outside tempera- 

 ture, for the just previous compression had heated it 4" 

 above the air, and the expansion could not cool it down before 

 5 seconds had elapsed. Whatever may have been the cause 

 of this rise, there is one point about which there is not the 

 slightest doubt, and this is the principal point that we are to 

 consider. Under all conditions of slow or rapid compression 

 and expansion, the final cooling after explosion was almost 

 identically the same, whether moist or dry air were used. 

 This was determined by the thermometer; and in this ex- 

 periment it must be admitted that the lagging of the ther- 

 mometer had no influence, for it would be precisely the 

 same in both moist and dry air. The rise of the gauge after 

 explosion with dry air was slightly greater than with moist 

 air, but this may have been due to a difference in the whirls 

 which the explosion always produced inside the jar. It 

 seems almost incredible that this fatal slip should have oc- 

 curred at such an extremely critical point in Espy 's work; 

 and I am impressed with the conclusion thus reached, not so 

 much by my interpretation of Espy's doubtful results, but far 

 more because the two sets of experiments dovetail into each 

 other so perfectly, and the one serves as a check on the 

 other. 



It is not a little remarkable that we have obtained some- 

 what the same result as this by another method of reason- 

 ing. The condensation of the moisture or the appearance 

 of the cloud, even in thoroughly saturated air, is exceedingly 

 evanescent in a jar of this kind. It is next to impossible, 

 except with very high compression (15 to 20 inches), to get 



