Variation in the Pressure of Saturated Vapours, 5 1 



there is no similarity between two pairs of these magni- 

 tudes. And so it should be, because equation (5) can only 

 be applied with the observation of certain conditions, which 

 generally speaking are not fulfilled. Nevertheless, it is possible 

 with nearly all liquids to find a temperature T which has 

 such properties that the magnitudes x and y are approximately 

 the same whatever three temperatures are taken, if only they 

 are adjacent to T and to each other. We will now turn our 

 attention to determining this temperature. I have discovered 

 two methods, which are here given, and for the sake of clear- 

 ness applied to aqueous vapour. 



6. Let us take three adjacent temperatures, T l5 T , and T', 

 differing for instance by 5°, so that T'— T =T — 1\ = 5, and 

 calculate the values of x and y. Then let us calculate their 

 values for three other temperatures as near as possible to the 

 first ; and then for three more temperatures, and so on ; and 

 thus combine a more or less considerable number of observa- 

 tions. We thus obtain values for x and y which are different, 

 but with the majority of substances experimented upon (i. e. 

 for which there are tables of vapour-pressures, in a state of 

 saturation, x, varying uninterruptedly with the corresponding 

 temperatures) give a maximum or minimum value. If the 

 mean (intermediate) of the temperatures T be laid along the 

 axis of abscissae, and x along the axis of ordinates, then a curve 

 is obtained with a maximum or minimum. There are, how- 

 ever, vapours for which the curve of x's continuously recedes 

 from or approaches the axis of abscissae with a rise of tem- 

 perature ; the path of the curve indicates where the maximum 

 or minimum lies, beyond the greatest or least of the tempera- 

 tures observed. 



The value of c— -ci varies very inconsiderably within small 

 limits of temperature; therefore, the magnitude x should 

 remain constant, whatever combination of temperatures be 

 taken, at temperatures near to that at which the vapour has 

 the properties of a perfect gas, and when in general the above 

 theory stands good ; and this is only possible when x has a 

 value near to its maximum or minimum"*, because a con- 

 siderable variation in T then produces an inconsiderable 

 variation in x. Hence the desired temperature T is that at 

 which x has a maximum or minimum value. We may here 

 remark that at this time y varies with the temperature, as is 

 seen from equation (4) ; or, more accurately, the increment 

 of y is equal to x multiplied by the increment of temperature. 



* I do not consider the case when the curve has an inflexion whose 

 direction is parallel to the axis of abscissae, because I have not met with 

 such an instance in my investigations upon vapours. 



E2 



