404 
rOPULAR SCIENCE REVIEW. 
has been freely charged with the vapour, these particles can be 
seen under a magnifier. This liquid must be a product of some 
chemical change of the vapour, and partial evidence of this 
change has been obtained by Dr. Tyndall. But it does not of 
course follow that because rays of light cause a chemical change 
in a vapour that a cloud must form. This formation involves 
the farther condition, that one, at least, of the substances pro- 
duced by the change shall have a vapour of such low tension 
at the temperature of the tube, as that, its point of maximum 
density being soon reached, the rest of it precipitates in the 
liquid form. Hence, the more attenuated the vapour with which 
the tube is charged, the slower the cloud of the substance produced 
by the change is in forming ; the first formed portions of the sub- 
stance preserving the state of (invisible) vapour, and the cloud 
beginning to form only when the point of maximum density of 
the vapour is reached. This is further shown by the fact that 
the cloud begins to form in the vapour next the light, and then 
extends from it, with a velocity depending upon the quantity of 
the vapour present. 
The chemical change which is the cause of the formation of 
the cloud is due to those rays of light which are active in 
causing other chemical changes, such as those upon which 
photography, sun-bleaching, &c., depend. In proof of this. Dr. 
Tyndall found that the rays of the electric-lamp were equally 
effectual in causing the clouds after the heat-rays had been 
intercepted by passing the beam through a solution of alum 
(which absorbs heat-rays without affecting luminous rays). 
Further, by passing the light through red, yellow, and blue 
glasses, he found that the first two intercepted most of the rays 
capable of bringing about the change, and that the blue glass 
did not do so. The blue rays, and those beyond them in the 
spectrum — that is, the most refrangible rays — are those which 
have hitherto been found to be those in which principally reside 
the power of causing chemical changes. 
Now, to return to the optical properties of these actinic 
clouds. The weight of substance sufficient to form a luminous 
white cloud is of the utmost minuteness. For example. Dr. 
Tyndall took a small bit of bibulous paper, rolled it up into a 
pellet not the fourth part of the size of a small pea, and 
moistened it with a liquid possessing a higher boiling-point than 
that of water ; he liehl the pellet in his fingers till it had 
become almost dry, then allowed dry air to pass over it in a 
connecting piece into the tube for a preliminary experiment; 
this experiment over, the pellet of paper was removed, and a 
current of dry air passed through the connecting piece and 
exficrimental tube ; then, through the same connecting piece, 
hydro^ddoric acid gas was passed into the tube, and found, in its 
