266 FRAGMENTS OF SCIENCE. 



pable of being understood, a very beautiful effect which, 

 under favorable circumstances, might be observed in our 

 atmosphere. This experimental tube contains an inch of 

 the iodide-of-allyl vapor, the remaining 29 inches necessary 

 to fill the tube being air, which has bubbled through aque- 

 ous hydrochloric acid. Besides, therefore, the vapor of 

 iodide of allyl, we have those of water and of acid within 

 the tube. The light has been acting on the mixture for 

 some time, a beautiful incipient blue cloud being formed. As 

 before stated, the " incipient cloud " is wholly different in 

 texture and optical properties from an ordinary cloud ; but 

 it is possible to precipitate in the midst of the azure the 

 aqueous vapor so as to cause it to form in the tube a cloud 

 similar to the clouds of our atmosphere. An exhausted 

 vessel of about one-third of the capacity of the experi- 

 mental tube is connected with it, the passage uniting both 

 being closed by a stop-cock. On opening this cock the 

 mixed air and vapor rush from the experimental tube into 

 the empty vessel ; and, in consequence of the chilling due 

 to rarefaction, the vapor in the experimental tube is pre- 

 cipitated as a true cloud. What is the result ? Instantly 

 the centre of the system of colored rings becomes bright, 

 and the whole series of colors corresponding to definite ra- 

 dial distances, complementary. While you continue to 

 look at the cloud, it gradually melts away as an atmos- 

 pheric cloud might do in the azure of heaven. And there 

 is our azure also remaining behind. The coarser cloud 

 seems drawn aside like a veil, the blue reappears, the first 

 ring-system, with its dark centre and correspondingly col- 

 ored circles, being restored. 



Thus patiently you have accompanied me over a piece 

 of exceedingly difficult ground ; and I think, as a prudent 

 guide, we ought to halt upon the eminence we have now 

 attained. We might go higher, but the bowlders begin 

 here to be very rough. At a future day we shall, I doubt 



