NATURAL PHILOSOPHY. 157 







afterward dried, the cake was friable; and when the dry powder 

 was pressed into a mould by means of hydraulic pressure, the cake 

 split up into laminse when subjected to the gases. After many exper- 

 iments with the materials in different proportions, it was found that 

 sulphate of lime one part, and calcined magnesia two parts, mixed 

 with water and moulded into a cake and dried, produced the best 

 results. This, however, is not all that could be desired, as in time 

 the cake becomes cracked and fissured by the gas. The illumin- 

 ating power is to that of lime, pressure and volume of gas being 

 equal, as fifty-four is to twenty-seven. 



BURNING GUNPOWDER IN VACUO. 



M. Bianchi, of Paris, has recently exhibited before the French 

 Academy some curious experiments on the combustion of gunpowder 

 in a vacuum. He found that this substance, and also the fulminates, 

 burnt quickly if loose in an exhausted vessel, and suddenly brought 

 to a temperature exceeding 2,000. If, however, the powder was 

 placed, under similar circumstances, in a pistol, it inflamed with the 

 suddenness exhibited in the air. Gun-cotton slowly disappeared, 

 the layer nearest the source of heat going first, but without the pro- 

 duction of any light. In all these cases the products of combus- 

 tion were the same as in air. Combustion also took place in nitro- 

 gen, carbonic acid, and other gases which do not support it, and there 

 was little diminution of the ordinary rapidity of the process. 



EFFECTS OF FROST ON IRON. 



Mr. David Kirkaldy, of Glasgow, in a recently published work de- 

 tailing his experiments in testing the strength of iron and steel, also 

 describes some experiments to test the effects of frost upon metal. 

 A bar of Glasgow best bar iron, of three-fourths of an inch diameter, 

 was forged into ten bolts, and six of them were exposed all night to 

 intense frost in the month of December, I860, then tested next morn- 

 ing, when the thermometer stood at twenty-three degrees Fah. The 

 other four bolts were kept warm all night, and protected during test- 

 ing. Three of the ten bolts were tested with gradual, and seven of 

 them with sudden strains. With gradual strains the bolts exposed to 

 frost gave way with 54,385 pounds strain ; the unfrozen bolts stood a 

 strain of 55,717 pounds, a difference of 2.3 per cent, in favor of 

 the latter. When submitted to sudden strains, the difference was 

 3.6 per cent, in favor of the unfrozen bolts. 



MEASURING DISTANCES BY THE TELESCOPE. 



At a recent meeting of the London Institution of Civil Engineers, 

 Mr. W. Bray gave the following description of an arrangement for 

 measuring distances by the telescope : He found that it was con- 

 venient to have two distinct hairs on the diaphragm of the level 

 one about three-twentieths of an inch above the level hair, and the 

 other as much below, so as to read one foot on the staff at one chain, 

 and ten feet at ten chains. Since, however, in focusing the instru- 

 ment to any object it was necessary to bring the cross hairs into such 



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