136 FKAGMENTS OF SCIENCE. 



suspended in our atmosphere ought, he supposed, to 

 generate the serenest skies. Newton does not appear 

 to have bestowed much thought upon this subject ; for 

 to produce the particular blue which he regarded as 

 sky-blue, thin plates with parallel surfaces would be 

 required. The notion that cloud-particles are hollow 

 spheres, or vesicles, is prevalent on the Continent, but it 

 never made any way among the scientific men of Eng- 

 land. De Saussure thought that he had actually seen 

 the cloud-vesicles, and Faraday, as I learned from 

 himself, believed that he had once confirmed the observa- 

 tion of the illustrious Alpine traveller. During my long 

 acquaintance with the atmosphere of the Alps I have 

 often sought for these aqueous bladders, but have never 

 been able to find them. Clausius once published a 

 profound essay on the colours of the sky. The assump- 

 tion of small water drops, he proved, would lead to 

 optical consequences entirely at variance with facts. 

 For a time, therefore, he closed with the idea of vesicles, 

 and endeavoured to deduce from them the blue of the 

 firmament and the morning and evening red. 



It is not, however, necessary to invoke the blue of 

 the first order to explain the colour of the sky ; nor is 

 it necessary to impose upon condensing vapour the diffi- 

 cult, if not impossible, task of forming bladders, when 

 it passes into the liquid condition. Let us examine 

 the subject. Eau-de-Cologne is prepared by dissolving 

 aromatic gums or resins in alcohol. Dropped into 

 water, the scented liquid immediately produces a white 

 cloudiness, due to the precipitation of the substances 

 previously held in solution. The solid particles are, 

 however, comparatively gross; but by diminishing the 

 quantity of the dissolved gum, the precipitate may 

 be made to consist of extremely minute particles. 

 Briicke, for example, dissolved gum-mastic, iu certain 



