SCIENTIFIC SUMMARY. 
95 
on the filter. The mass on the filter must in this case be treated with 
alcohol, which will dissolve the fuchsine, whilst the natural colouring 
matter of the wine remains insoluble. M. Didelot’s method is still more 
simple. He places a little ball of gun-cotton in a test-tube with some of the 
wine to be examined, shakes the whole strongl} r for a short time, and 
carefully washes the cotton in several waters; if the wine be pure, the 
cotton becomes white again, but the tint given to it by artificial colorants 
persists. The actual substance employed may generally be recognized by 
the addition to the coloured cotton of a few drops of ammonia. The 
colour is discharged when fuchsine has been used, rendered violet when the 
colouring matter is orchil, and greenish when it is elder juice. 
Atomic Weight of Selenium. — By an investigation of numerous selenium 
compounds, MM. Pettersson & Ekman have endeavoured to ascertain the 
precise atomic weight. As a mean of seven direct analyses, they obtained 
the number 79-01. By reducing selenous acid by sulphurous acid, and col- 
lecting and drying the precipitate, the number 79-08 was obtained as the 
mean of five determinations, and this they believe to be very nearly correct. 
(Berlin Chemical Society, Sept. 1876.) 
Physical Properties of Gallium. — M. Boisbaudran has prepared a small 
quantity of nearly pure gallium, and finds that its melting point is about 
29*5° C. (==85*1°F.), so that it is fused by the heat of the hand. When 
once liquefied, it exhibits the phenomenon of surfusion in a very remarkable 
manner, remaining liquid for more than a month as a globule capable of 
being divided and reunited by the blade of a knife, in a room of which the 
temperature often fell below the freezing-point. By contact with a piece of 
solid gallium, it was immediately solidified. It crystallizes readily, only 
oxidizes at the surface when heated to redness, and does not volatilize. It 
density is about 4-7, so that in this, as in some other respects, it stands be- 
tween aluminium and indium. (“ Journ. de Phys.,” Sept. 1876.) 
Liquid Carbon Dioxide in Mineral Cavities.— On heating a microscopic 
slide of quartz containing fluid cavities only to a moderate temperature, Mr. 
Hartley was surprised to find that the liquid, previously perfectly visible 
under the microscope, had disappeared. On cooling, the liquid re-appeared, 
accompanied by a sort of flickering movement within the cavity. Experi- 
ments on fluid cavities in various minerals made by Brewster in 1823, 
showed that the liquids all disappeared below 88° F., that their expansion 
between 50° and 80° F. was 32 times that of water, and their index of 
refraction P2946 in topaz and 1*2106 in amethyst. From these results 
Simmler, and later, Sorby and Butler, concluded that the liquid must be 
carbon dioxide. The author sought carefully to determine the critical point 
of the liquid, which he did by immersing the slide in water of known tem- 
perature, removing, wiping hastily, and examining. As a result, it appeared 
that the critical point lay between 30'7-5 o and 31° C., that point for pure 
carbon dioxide having been fixed by Andrews at 30 - 92° C. In further cor- 
roboration qf this view is the fact that when water was also present in the 
same cavity, the other liquid floated on it ; the density of carbon dioxide 
being 0 83 at 0° 0. and 0-6 at 30° 0. Moreover, Geissler has shown the 
presence of this gas in quartz by its spectrum in a vacuum tube in which the 
quartz was broken. In explanation of the formation of these fluid cavities, 
