148 Royal Society : — 



cub. centims., or, applying the above coefficient of expansion of glass, 



59-7300 cub. centiins. at 4°. The mean of this value and that 



,^ . ,. ,^, 59-7300 + 59-7323 ,^^^,, ^ 

 obtained m (2), = -^ = 59'7311 cub. centims., was 



taken to represent the true capacity of the vessel at 4°. 



(6) The freezing-mixtures used were prepared by cooling com- 

 mercial hydrochloric acid (sp. gr. =1140) in the snow out of 

 doors, the temperature of which, as well as of the air, was on 

 the first day about —9° C, but on subsequent days rose to about 

 — 5°, mixing equal weights of this cooled acid and of snow, using 

 separate portions of this first mixture to cool more acid and snow, 

 and finally bringing together these last. 



It soon appeared that httle advantage was gained by trying to 

 cool the snow, on account of its very low conducting-power in such 

 a loose porous condition ; and in the later experiments the tempera- 

 ture of the acid alone was lowered before the final mixture with 

 snow. The glass vessels containing the mixtures were large enough 

 to maintain the cold required for a long time, and steadiness of 

 temperature was secured by surrounding them on all sides with a 

 layer of cotton wadding, kept in place by stiff brown paper, and 

 by conducting all the operations out of doors in the unusually 

 cold atmosphere prevailing at the time. 



(7) In determining the temperature of the freezing-mixtures an 

 alcohol thermometer was used, graduated to single degrees, and ad- 

 mitting of half a degree being read ; but the scale being found by 

 no means accurate, its absolute readings were altogether discarded. 

 By comparison with a good mercurial thermometer at three or four 

 points between — 10° and -|-40° C, and calculation from Is. Pierre's 

 coefficients, the real length of a degree on the part of the stem 

 corresponding to — 40° was determined; and the temperature 

 of fusion of the mercury being accurately noted and assumed 

 = — 38°'85 C, as determined by Balfour Stew^art*, the addition 

 or subtraction of four or five degrees, as above obtained, gave all 

 the other temperatures observed. 



(8) The above weighings and all others to be mentioned were 

 made with an excellent balance by Becker, carefully adjusted and 

 tested at the outset. With a load of a kilogramme in each pan a 

 difference of weight of -J^ milligramme can be detected, and ^ 

 milligramme may be fully relied upon. All weighings were reduced 

 by calculation to the corresponding results in vacuo, the temperature 

 and pressure of the atmosphere being noted on each occasion ; 

 and the results quoted are those thus corrected. ^ 



(9) The specific-gravity flask was now filled with alcohol (at one 

 time absolute, but which, by long keeping in the laboratory and 

 occasional opening of the bottle, had absorbed some moisture, and 

 w^as really about 95 or 96 per cent.), and three weighings were 

 obtained after the liquid had been carefully adjusted to the mark 

 at temperatures close to the freezing-point of mercury t. 



* With an air- thermometer (Proc. Eoj. Soc. 1863, vol. xii. p. 674). 

 " t The alcohol, as afterwards mercury, was brought to near the required 



-*r- 



