116 FRAGMENTS OF SCIENCE 



medium rendered slightly turbid by the mechanical 

 suspension of exceedingly small foreign particles. 



Here, as before, we encounter our sceptical ' as if.' 

 It is one of the parasites of science, ever at hand, and 

 ready to plant itself and sprout, if it can, on the weak 

 points of our philosophy. But a strong constitution 

 defies the parasite, and in our case, as we question the 

 phenomena, probability grows like growing health, 

 until in the end the malady of doubt is completely 

 extirpated. The first question that naturally arises is 

 this : Can small particles be really proved to act in the 

 manner indicated ? No doubt of it. Each one of you 

 can submit the question to an experimental test. 

 Water will not dissolve resin, but spirit will dissolve 

 it ; and when spirit holding resin in solution is dropped 

 into water, the resin immediately separates in solid 

 particles, which render the water milky. The coarse- 

 ness of this precipitate depends on the quantity of the 

 dissolved resin. You can cause it to separate either 

 in thick clots or in exceedingly fine particles. Professor 

 Briicke has given us the proportions which produce 

 particles particularly suited to our present purpose. 

 One gramme of clean mastic is dissolved in eighty- 

 seven grammes of absolute alcohol, and the transparent 

 solution is allowed to drop into a beaker containing 

 clear water, kept briskly stirred. An exceedingly fine 

 precipitate is thus formed, which declares its presence 

 by its action upon light. Placing a dark surface be- 

 hind the beaker, and permitting the light to fall into 

 it from the top or front, the medium is seen to be 

 distinctly blue. It is not perhaps so perfect a blue as 

 may be seen on exceptional days among the Alps, but 

 it is a very fair sky-blue. A trace of soap in water 

 gives a tint of blue. London, and I fear Liverpool, 

 inilk makes an approximation to the same colour, 



