432 



HYDRODYNAMICS. 



Specific It is obvious, however, that all these measures are 

 Gravities. a ff ec ted by a variation in the density, the temperature, 

 ^~*~*^" ' and the humidity, of the external atmosphere. The 

 weight, too, of the gases, when they are introduced 

 into the receiver, is affected by the temperature and 

 pressure of the air. The contraction and dilatation of 

 the glass vessel requires also to be computed ; and the 

 weight of the gas itself is affected by the tempera- 

 ture and the degree of drying which it has experienced. 

 These various sources of error likewise affect the results, 

 in so far as they affect the external atmospherical air in 

 which both the air itself and the gas must be weighed. 

 Some allowance must also be made for the imperfect 

 exhaustion of the glass vessel, which is always visible 

 by its effect upon the barometer. 



It will readily be seen, that it must require no small 

 degree of trouble to calculate the combined influence 



of these different causes, though, in order to obtain accu- 

 rate results, such a calculation becomes absolutely ne- 

 cessary. As it would be impracticable in the present 

 article to enter into any lengthened examination of the 

 subject, we must refer such of our readers as wish to 

 study it profoundly, to the l.Qth, 2()th, and 21st chap- 

 ters of M. Riot's valuable work entitled, Traite de Phy- 

 sique, which not only contain the method of deducing 

 the necessary formulae, but also many excellent remarks 

 and suggestions which could only have been given by 

 one who had investigated the subject both theoretically 

 and practically. The following are his principal formulae, 

 which are suited to a temperature of 32 of Fahren- 

 heit, or that of melting ice, and to a state of the at- 

 mosphere when the barometer stands at 0.76 metres, or 

 29.94 English inches. 

 In these formulae, 



At the time when the 

 glass vessel is weigh- 

 ed empty. 



At the time of the intro- f p 

 duction of the gas into ) t' 

 the glass vessel. \_p' 



fP" 

 At the time when the 



glass is weighed, full 

 of gas. 



When the glass vessel is f 

 weighed empty a se- I P'" 

 cond time, after it I p'" 

 has been weighed full I t"' 

 of gas. 



fP" = 



\P" = 



r " = 



ih" = 



1. Formula suited to the case where the Gases are perfectly dry. 

 P''_P(y.76 



\~ * 



2Y= 



(No. 1.) 



1 + <".6.00375' 1-H- 0,00375" 1+/ / ".0,00375 (N o . 2.) 



I -1 Y 7- j \ __ 



V L I *" )lr _|_ ^ * *T * * /f' \ ' // 



1 + t. 0,0037 5 1+<'.0,00375 1 + ^".0,00375 



l+r.0,00375 



Specific' 

 Gravities. 



X the absolute weight of atmospheric air contained in the glass vessel at a tempe- 

 rature of 32, and under a pressure of 29.9* inches of mercury, as calculated 

 from the formulae. 



the absolute weight of any gas under the same circumstances. 



the interior volume of the glass vessel at the same temperature. 



the cubical dilatation of glass for every degree of, the centigrade thermometer. 



the absolute weight of the glass vessel, which never changes. 



the atmospherical pressure. 



the temperature of the air. 



the state of the hygrometer. 



the tension in the interior of the glass vessel, after a vacuum is made by the air 

 pump. 



external pressure exerted upon the gas. 



temperature of the gas. 



its hygrometric state. 



the weight of the glass vessel filled with gas. 



the atmospherical pressure. 



the temperature of the external air. 



the state of the hygrometer. 



: the weight of the glass vessel empty observed in air 

 : atmospherical pressure. 

 : temperature; 



In the ordinary state of the atmosphere, the barome- 

 ter and thermometer indicate only very small and pro- 

 gressive changes, so that in the short time which can 

 elapse between the different weighings of the gas, 

 we may safely suppose the atmospherical pressure p", 

 and the temperature t", corresponding to the interme- 

 diate weighing of the glass vessel, as arithmetical 



means between the extreme pressures p, p'", and the 

 extreme temperatures t, t'". In proportion, therefore, 

 as the variations in these elements have been inconsi- 

 derable, we may consider them as compensating them- 

 selves in the terms of X : These terms will consequently 

 disappear, and the formula will be reduced to the fol- 

 lowing simple form : 



,_ (P + P'") \ 



Y = 



This formula will be found sufficiently exact when 

 the gases and the atmospherical air are perfectly dry ; 

 but as this is never the case, and as the aqueous vapour 



(1 + f. 0,00375 ).0'",76 



(No. 3.) 



has a very considerable influence upon the weight at 

 a temperature above 50 of Fahrenheit, it is necessary 

 to compute its effect. 



4- 







