Chemistry. 445 
When potassium or sodium is wrapped up in platinum-foil, 
and introduced into the flame of a spirit-of-wine lamp, it bursts 
into flame, and perforates the platinum-foil. 
When potassium is heated on a slip of platinum-foil, and is 
withdrawn in a state of fusion, it suddenly ignites the platinum. 
If we fold up a fragment of concrete potassa in platinum-foil, 
and it becomes anhydrous by exposure for some time in the spirit 
lamp, it afterwards burns with scintillation, which seems to be 
owing to the reduction of the potassa^ and consequent evolution 
Sometimes when the glowing mass of antimony and platinum 
falls down, it continues to hum like a taper for a short while. 
A little powdered native nickel from Kfesse, or the same in 
small fragments, folded up in platinum-foil, gave repeated slight 
explosions like the cracks of a whip ; the particles then perf©, 
rated their envelope, like a sieve, and rolled down through the 
flame like little fire-balls, in a state of intense ignition. 
32. Composition of Rhuharh, — Mr Brande has found, that the 
root of the Rheum palmatum is composed as follows : 
Water, • - • 8,2 
Gum, - - , - 31.0 
Resin, - - , - . 1 0.0 
Extract, Tan and Gallic Acid, - - 76.0 
Phosphate of Lime, - - 2.0 
Malate of Lime, - - - 6.5 
Woody Fibre, - - - 16.3 
100.0 
Quart. Journ. vol. x. p 201. 
33. On the Colouring Matter of the Shell of the Crab. — It is 
well known, that when a crab is boiled, its shell assumes a fine red 
colour, the nature and origin of which have hitherto been un- 
known. At the desire of M. Latreille, a series of experiments 
upon it was undertaken by M. J. L. Lassaigne. The shell of 
the crab having been carefully freed from all fleshy matter, was 
plunged in pure alcohol, of the temperature of 59° Fahr. They 
assumed a scarlet colour, which was instantly communicated to 
the alcohol. The alkoholic solution of the colouring matter was 
then decanted, and new doses of alcohol added, till it ceased to 
be coloured. The shells thus exhausted lost their property of 
becoming red in boiling water. From the spontaneous evapo- 
of potassium 
