358 Mr. C. Tomlinson on the Motion oj 



saturation of the space at a temperature of 0°, although the 

 aperture of the tube had a diameter of 1*1 Paris lines. 



We are incontestably entitled to conclude from these experi- 

 ments that Dalton's theory, in so far as it assumes that the 

 air and the vapour existing in the same space are independent 

 of each other, is totally unfounded ; the true view rather is that 

 the air exerts a pressure upon the vapour and the vapour upon the 

 air. I make use here of this mode of expression merely in order 

 to represent the effect. I hope at a future opportunity to be able 

 to show that the humidity must be regarded as adhering to the 

 molecules of air, and that the phenomena admit of a simple 

 explanation by means of a natural hypothesis concerning the 

 expansion of dry and wet molecules of air. 



If it be desired to apply the theory developed in the foregoing 

 to the circumstances of the aqueous vapour in the atmosphere, 

 it is in the first place to be inferred from it that (since the 

 diffusion of the vapour in the air takes place but very slowly, 

 and since in different places, according to the temperature and 

 the magnitude of the surface of the water exposed to the air, 

 very different quantities of vapour pass into it) in regard to the 

 humidity of the air, strictly speaking, no relations subsist con- 

 formable to any law. Of course the continually existing cur- 

 rents of air occasion a complete mingling of the more and less 

 dry masses of air ; but this takes place in no uniform manner, 

 and therefore no exact relation of dependence exists between 

 the degrees of humidity in different points of any space. But 

 particularly the idea of an atmosphere of vapour subsisting in- 

 dependently by itself appears to be inadmissible, and the data 

 furnished by the psychrometer can no longer be regarded in 

 any other light than as the expression of local humidity. 



XLVII. On the Motion of Camphor towards the Light. By 

 Charles Tomlinson, Lecturer on Physical Science, King's 

 College School, London*, 



IN ChaptaPs Elemens de Chimie (vol. i. p. 36), published in 

 1790, a property is claimed for light which has been recog- 

 nized by later philosophers. Por example, in Professor DanielPs 

 'Introduction to the Study of Chemical Philosophy' (2nd ed., 

 1843, p. 452), we read, "Light is capable of acting upon and di- 

 recting homogeneous attraction, and of influencing the crystalliza- 

 tion of certain substances. Evidence of this may very commonly 

 be found in druggists' shops, where, in the glass jars which con- 

 tain camphor and are placed in the windows, beautiful crystals 



* Communicated by the Author, having been read at the British"Associa- 

 tion at Cambridge, October 1862. 



