74 



Prof. J. W. Mallet on the 



[Mar. 15, 



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

 hydrochloric acid (sp. gr. =1140) in the snow out of doors, the tem- 

 perature 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 little 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 temperature of the acid alone 

 was lowered before the final mixture with snow. The glass vessels con- 

 taining the mixtures were large enough to maintain the cold required for 

 a long time, and steadiness of temperature wa3 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 admitting 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.j and calculation from Is. Pierre's coefficients, the real length of a de- 

 gree on the part of the stem corresponding to —40° was determined ; and 

 the temperature of fusion of the mercury being accurately noted and as- 

 sumed = — 38°-85 C, as determined by Balfour Stewart*, 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 y 1 ^ milligramme can be detected, and ^ milligramme may be fully re- 

 lied upon. All weighings were reduced by calculation to the correspond- 

 ing results in vacuo, the temperature and pressure of the atmosphere 

 being noted on each occasion ; aud the results quoted are those thus cor- 

 rected. 



(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 was 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 f . 



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



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

 ture before introduction into the final freezing-mixture, and a separate small portion in 

 a tube was similarly cooled, to be used in filling up to the mark if necessary. The 

 stopper was carefully inserted as soon as the adjustment of the liquid was secured, so 

 as to avoid any loss by evaporation. 



