70. TO SOFTEN STONE.--Marble or granite may be deprived in some measure, of the property of cohesion by being heated red hot and then quenched in oil. In this case, the carbonic acid which constitutes the cohesive property of the stone, is expelled by the heat; and the vacuum thus produced in its pores, are in some measure, filled by the oil by the pressure of the atmosphere; by which means the stone acquires a texture quite different from what it had previously. This however, is not often applied to any valuable purpose. 71. TO CHANGE WOOD APPARENTLY, TO STONE.--Provide a block, or plank of soft wood, of the dimensions required, and give it two or three coats of linseed oil, allowing each to dry. Then having prepared some pieces of marble or granite as directed in the last experiment, pulverize them to a gross powder; brush over the wood with a heavy coat of copal varnish (see 47) mixed with an equal quantity of venice turpentine; let this rest about an hour, and then strew the stone powder over every part of it, so as to cover the surface completely. If marble is to be imitated, the powder of different colours, especially the white and blue, may be prepared separately, and may be strewed on the work in such shades as will appear the most natural. Granite may also be crossed or striped occasionally with streaks of a coarser grain, which will give it a very deceptive effect. When the varnish is thus covered with stone, a heavy roller, or round log of wood, having a blanket folded and wrapped round it, should be rolled over the work, that the larger grains, (which of course will be the most exposed,) may the more firmly adhere. In this manner, a very perfect imitation of stone may be given, and the wood thus prepared will be exceedingly durable, and will answer for many purposes, as well as real stone. 72. TO RENDER WOOD, CLOTH OR PAPER FIRE-PROOF.--Dissolve one ounce of alum, half an ounce of sub-borate of soda and half an ounce of cherry tree gum, in half a pint of vinegar. Dip any cloth or pieces of paper, or wood, in this mixture and let them dry;--they cannot afterwards be ignited so as to blaze, but may be considered safe with regard to their taking fire by accident. _Note._--Though this composition is a very powerful preventive against fire, it is too complex for common use, and has too much colour for white cloths or papers; but a solution of one ounce of sub-borate of soda in a pint of water is very transparent and harmless, and will answer in most cases nearly as well. 73. TO PRODUCE FIRE READILY.--_Process 1._ Mix together gently but intimately, two or three grains of chlorate of potass, and an equal quantity of loaf sugar, both previously reduced to fine powder:--dip the end of a strip of glass, or a straw in sulphuric acid, and with it gently touch the powder,--it will instantly burst into flame.