160 



topaz contained titanium of a scarlet colour and transparent. These had 

 seven different forms, some of them very curious, which are figured. 



6. On Crystals and Cavities in Garnet. — These are very fre- 

 quent, and exhibit very singular phenomena hy polarized light. The 

 cavities are sometimes surrounded by spaces or sectors of polarized 

 light, indicating that the garnets had been soft after assuming their 

 present forms. 



In one case, a liquid or gas had escaped from a cavity, and had 

 left circular crystals of singular beauty. 



2. On the Absolute Zero of the Perfect Gas Thermometer ; 

 being a Note to a Paper on the Mechanical Action of 

 Heat. By W. J. Macquorn Rankine, Esq. 



Temperature being measured by the pressure of a perfect gas at 

 constant density, the absolute zero of temperature is that point on 

 the thermometric scale at which, if it were possible to maintain a 

 perfect gas at so low a temperature, the pressure would be null. 



As no gas is entirely devoid of cohesion, the immediate results of 

 experiment give only approximations to the position of this absolute 

 zero. These approximate positions approach nearer to the true 

 position as the gas is rarefied. 



The author having deduced the true position of the absolute zero 

 from M. Regnault's experiments on atmospheric air and carbonic 

 acid, soon after their publication, announced the result in the Edin- 

 burgh New Philosophical Journal for July 1849, and in the Trans- 

 actions of the Royal Society of Edinburgh, vol. xx. 



The present paper gives the details of the method of determina- 

 tion which he adopted, and a copy of the diagram which he used. 



The following were the results arrived at : — 



The absolute zero of the perfect gas thermometer is 



274°- 6 centiojrade, or 1 , , , ,, , . „ , . 



° > below the temperature oi melting ice. 



494°-28 Fahrenheit, J ^ ^ 



The coefficient of expansion of a perfect gas, in fractions of its 



volume at the temperature of melting ice, is consequently, — 



Per degree of the centigrade scale, ——— = 0-00364166. 

 Per degree of Fahrenheit's scale, -r^^.yr.- 0-00202314. 



