MECHANICS AND USEFUL ARTS. 37 



ten per cent, of water at 212, and nine more at 250, or only nineteen 

 per cent, at temperatures not dangerous to the contents of the safe. 



In most fires the exposure is for so brief a period that the protection 

 in some of the best safes is adequate ; but there is the constant possi- 

 bility that the fire may be too powerful and too protracted for the com- 

 position employed, and the protection consequently inadequate. 



Can this protection against fires be increased ? The incombustible 

 inclosure is of wrought iron, and nothing could be better than this. 

 The points, therefore, remaining for consideration are, What is the 

 best practicable non-conductor ? and, What is the best composition for 

 keeping down heat for vaporization ? 



In answer to the first question, experiments were made, among other 

 materials, with Infusorial Earth ; a mixture of Sal Soda and Gypsum ; 

 a mixture of Glaubers Salt and Gypsum; set Cement; Alum and dry 

 Cement ; Gypsum with Gelatine. 



A wrought-iron cup, containing about eight ounces of water, was filled 

 with each substance in its turn, and the bulb of a thermometer imbed- 

 ded to the same distance from the bottom in all. The vessel and its 

 contents were then subjected to the same degree of heat. The con- 

 ducting power, or the facility of heating throughout, was measured by the 

 number of degrees swept over by the ascending column of mercury in 

 successive minutes. The range of heat was from 220 to 572. 



Infusorial earth was heated 27 in one minute. 



Sal soda and gypsum taken in equal parts, 14 in one minute., 



Glaubers salt and gypsum, taken in equal parts, 12 in one minute. 



Cement, set and dried, 11 in one minute. 



Potash alum, 3 oz. ) 5 per minute, from 220 to 300. 



Dry cement, 6 oz. ) 4 per minute, from 300 to 572. 



Gypsum, 6 oz., and water, 6 oz., | 2 per min. from 220 to 300. 

 with 3 per cent, of gelatine, ) 4 per min. from 300 to 572. 



From each, the water due to a temperature of 212, had, as already 

 intimated, been driven out. In the cement, alum, and gypsum, there 

 remained water in combination. The infusorial earth proved the best 

 conductor, and would of course be the poorest substance for filling a 

 safe. Of all, the gypsum and gelatine, as prepared for this experiment, 

 .throughout the range in which the contents of the safe are secure 

 against fire, namely, below 300, affords the best protection, so far as 

 conduction is concerned. It is, indeed, difficult to conceive of a sub- 

 stance better suited for non-conduction than this mass of set plaster and 

 gelatine, after the surplus water alternating with every particle of gyp- 

 sum has been driven out, leaving behind an infinity of minute cavities, 

 rendering the whole porous and non-conducting to the last degree. 



How shall this quality be combined most advantageously with the 

 second requisite mentioned above, that of supplying matter to be va- 

 porized, thereby carrying the heat away ? 



Two sets of experiments were undertaken to determine this point. 

 The first on a small scale, the second on a large and more practical 

 scale. The first was conducted at the same time with the series al- 

 ready detailed, and employing the same apparatus. The wrought-iron 

 cup was filled with each mixture in turn, supported at a constant alti- 

 tude over a flame of uniform height, and the thermometer imbedded to 

 the same depth in all. 



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