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it is deprived of a large proportion of hydrogen, and nitrous 
acid takes its place. The consequence is, that so large a 
quantity of oxygen is contained in the compound, that the 
moment it is brought in contact with heat, the oxygen combines 
with the carbon and hydrogen, and an immense quantity of 
carbonic acid gas and vapour of water are formed, and an 
explosion is the result. 
Pyroxylin differs from gunpowder in many respects. In the 
first place, it ignites at a temperature of 400° Fahr., which is 
200° less than the degree of heat at which gunpowder explodes. 
Then its explosive force is three times as great as gunpowder. 
On this account it is not adapted for guns ; it blows them to 
pieces, and has, after all, a less propulsive effect on the ball. 
Nevertheless, for blasting, gun-cotton is very useful, and is 
much employed at the present day. It has also this advantage 
in mines — the gases it produces are less injurious than those 
produced by gunpowder. It resists the action of water, and, if 
plunged into it, or exposed to the action of damp air, it regains 
its explosive force by drying. 
Pyroxylin has many curious chemical properties, an ac- 
count of which my limited space will not permit me to 
give; but there is one so interesting, on account of its 
practical importance, that a natural history of cotton would 
hardly be complete without mentioning it. I allude to the 
solubility of gun-cotton in ether : it is insoluble in water 
and in alcohol, but it is soluble in ether. When dissolved thus, 
it is called Collodion. When collodion is spread on any surface 
and exposed to the air, the ether evaporates, and a delicate 
transparent film of pyroxylin is left. When iodide of potassium 
is added to collodion, and spread on a glass plate, and then 
dipped into a solution of nitrate of silver, an iodide of silver is 
formed, which, on being exposed to the action of light, under- 
goes a change of colour. By these means sun-pictures, formed 
by the camera, may be impressed upon the plate. These, 
properly prepared, become the negative plates by which the 
photographer multiplies, on sensitive paper, the pictures he has 
taken. Here again we trace the reward following upon diligent 
search into the properties of natural substances. 
The study of the chemical properties of the cotton-hair has 
not only led to the multiplication of natural pictures, but to 
the reproduction, at a cost which places them within reach of 
almost the poorest in the land, of the great works of genius 
which no mere copies of natural objects can equal. 
Before leaving the subject of the cotton-hair, there are one 
or two other points to which I would refer as worthy of notice 
and investigation. It is well known that vegetable fabrics 
receive different impressions from the colouring substances 
with which they are dyed. Cotton, hemp, and jute, when 
