2 Physics. 269 
Tn the course of the experiments on the chemical action of light 
which have been already comm regen ites in Beattie to the Royal 
of the air-pump itself were neal ployed for ain so ie 
Tt was found possible, by shutting off the residue of air and 
a 
cylinders of the pump, to obtain with some substances, and with- 
out ae the experimental tube, fifteen or twenty clouds in 
succes 
The al ido thus precipitated differed from each other in lumin- 
ous energy, some shedding forth a mild white eee others flash- 
ing out with sudden and surprising brilliancy. ifference of 
action i 1s, of course, to be referred to the different. ‘sellocnivi en- 
ergies of the particles of the clouds, oar were produced by sub- 
stances of very different refractive indic 
Different clouds, moreover, possess very "different degrees of sta- 
bility ; some melt away rapidly; while others linger for minutes in 
the ex pariuistital tube, resting upon its bottom as they tein 
through the experimental tube as if they were moving through a 
viscous mediu 
hin 
exhibited by some of these clouds; the colors are best aby 
looking along the experimental tube froma point isbove it, the face 
being turned. towards the source of illumination. The dif erential 
motions introduced by friction against the interior surface of 
pre ta tube often cause the colors to arrange themselves in distinct 
ayers, 
The difference in texture chee by different clouds caused 
me to look a little more closely than I had previously done into the 
ion, *y 
t be 
vapor may F be roceniedan 
@ number of polyhedra, the ps articles along the pamnaiag surfaces 
is 
of which move in posite directions when precipitation actually 
— ae c Sond particle has consumed a ee hedron of 
n its + fo rmation ; an p that the size of the 
