71 



where the rushing commences, and the pressure begins to vary, 

 which is also the temperature to which the fluid is reduced in the 

 other part of the tube before it reaches the end ; and H the quan- 

 tity of heat which must be taken away to fulfil this condition, during 

 the passage of a quantity of fluid of volume u', under a pressure 

 equal to p , at the temperature ^, through the apparatus. 



From this it follows, that the test of Mayer's hypothesis for any 

 particular temperature is to try whether, when the air enters at 

 that temperature, it leaves the rapids at precisely the same tempe- 

 rature. Calorimetrical methods of experimenting upon this appa- 

 ratus, like those of Joule, but susceptible of being continuously 

 used for any period of time, are suggested for determining, possibly 

 with very great accuracy, the value of 



\ E 



J fx{l+Et) 



for any temperature, should it not be exactly zero for all tempera- 

 tures, as it would be if Mayer's hypothesis were true. The value 

 of J having been determined by Joule with very remarkable accu- 

 racy, it follows that such experimental researches, besides affording 

 the solution of the problem which forms the subject of this paper, 

 would determine the values of Carnot's function, by an entirely new 

 method, for the temperatures of the experiments. 



Dr Gregory exhibited a specimen of a beautiful fibrous silky 

 white salt, taken about thii'teen years ago, by Donald Campbell, 

 Esq., from the joinings of the slabs of limestone forming the roof of 

 the highest of the chambers of construction, discovered by Colonel 

 Vyse above the King's Chamber in the great pyramid of Ghizeh. 

 No other part is lined with limestone, and there only this salt ap- 

 peared. Dr Gregory found it to be absolutely pure chloride of sodium, 

 60 pure, indeed, that it had not undergone the slightest change in 

 thirteen years, although only wrapped in paper. Had lime or 

 magnesia been present, it would have deliquesced. Under the 

 microscope, the fibres exhibited oblique angles and fractures, and 

 they may possibly be regular six-sided prisms, derived from the 

 cube. Dissolved in water, the salt crystallized by evaporation in 

 the usual form. When heated, it gave off a trace of water, but re- 



