118 FRAGMENTS OF SCIENCE 



2^ to 3 inches internal diameter. The vapor to be ex- 

 amined is introduced into this tube in the manner already 

 described, and upon it the condensed beam of the electric 

 lamp is permitted to act, until the neutrality or the ac- 

 tivity of the substance has been declared. 



It has hitherto been my aim to render the chemical 

 action of light upon vapors visible. For this purpose 

 substances have been chosen, one at least of whose prod- 

 ucts of decomposition under light shall have a boiling- 

 point so high, that as soon as the substance is formed it 

 shall be precipitated. By graduating the quantity of the 

 vapor, this precipitation may be rendered of any degree 

 of fineness, forming particles distinguishable by the naked 

 eye, or far beyond the reach of our highest microscopic 

 powers. I have no reason to doubt that particles may be 

 thus obtained, whose diameters constitute but a small 

 fraction of the length of a wave of violet light. 



In all cases when the vapors of the liquids employed 

 are sufficiently attenuated, no matter what the liquid may 

 be, the visible action commences with the formation of a 

 blue cloud. But here I must guard myself against all mis- 

 conception as to the use of this term. The "cloud" here 

 referred to is totally invisible in ordinary daylight. To 

 be seen, it requires to be surrounded by darkness, it only 

 being illuminated by a powerful beam of light. This blue 

 cloud differs in many important particulars from the finest 

 ordinary clouds, and might justly have assigned to it an 

 intermediate position between such clouds and true vapor. 

 With this explanation, the term " cloud, " or "incipient 

 cloud," or "actinic cloud," as I propose to employ it, 

 cannot, 1 think, be misunderstood. 



I had been endeavoring to decompose carbonic acid 



