SCIENTIFIC SUMMARY. 
215 
may be taken as representative of a great number of others. The specific 
gravity of this liquid is 0°-85, that of water being unity ; the specific gravity 
of its vapour is 3°*26, that of aqueous vapour being 0°-6. Now, as the size 
of the cloud-particle is directly proportional to the specific gravity of the 
vapour, and inversely proportional to the specific gravity of the liquid, an 
easy calculation proves that, assuming the size of the vapour polyhedra in 
both cases to be the same, the size of the particle of toluol cloud must be 
more than six times that of the particle of aqueous cloud. It is probably 
impossible to test this question with numerical accuracy ; but the compara- 
tive coarseness of the toluol cloud is strikingly manifest to the naked eye. 
The case is representative. In fact, aqueous vapour is without a parallel in 
these particulars,- it is not only the lightest of all vapours, in the common 
acceptation of that term, but the lightest of all gases except hydrogen and 
ammonia. To this circumstance the soft and tender beauty of the clouds of 
an atmosphere is mainly to be ascribed. The sphericity of the cloud-particles 
may be immediately inferred from their deportment under the luminous 
beams. The light which they shed when spherical is continuous ; but clouds 
may also be precipitated in solid flakes, and then the incessant sparkling of 
the cloud shows that its particles are plates and not spheres. Some portions 
of the same cloud may be composed of spherical particles, others of flakes, 
the difference being at once manifested through the calmness of the one por- 
tion of the cloud, and the uneasiness of the other. The sparkling of such 
flakes reminds one of the plates of mica in the River Rhone at its entrance 
into the Lake of Geneva, when shone upon by a strong sun.” — Paper read 
before Royal Society, March 8. 
A Form of Actinometer is described by Mr. Louis Bing in the Photographic 
News for February. It consists of a rectangular box, to one side of which 
a square tube is applied. At the aperture of the tube there is a slide with a 
rectangular opening, by moving which he can either admit light into or 
exclude it from the tube. One side of the tube is made of yellow non-actinic 
glass, and the opposite or interior side is made of white glass. By looking- 
through the yellow glass one can watch the action of the light in the tube 
by simple inspection. A scale is marked on the strip of white glass by 
means of a standard tint. The side of the box to which the tube is fixed is 
made to take off, and is held in its place by means of four little springs, 
like the back of a dark slide. A cylinder is placed vithin the box, against 
which the white glass of the tube is pressed, and which is surrounded with 
sensitive paper. The top of the box, which has a milled head, is also 
made to take off. This milled head is fixed to a rod which passes through 
the cylinder, and by means of which the photogi-apher can turn the cylinder 
either way. By unscrewing this little top-piece he can remove the milled 
head, and then the top of the box. The cylinder can also be lifted out 
of the box for the purpose of charging it with sensitive paper. This is done 
once, in the morning, for the work of the whole day. After inserting the 
cylinder the top of the box is replaced, the milled head and its little screw. 
Fix the side with the tube, and you have only for every fresh exposure to 
give a slight turn to the cylinder, by means of the milled head, in order to 
bring a fresh part of the paper forward, at the back of the white glass.” 
Flecti'o-Chemistry in Metallurgical Operations. — M. Becquerel has recalled 
