MECHANICS AND USEFUL AETS. 41 



On opening the furnace, the appearances of the safes had not appar- 

 ently changed since the examination at the close of the first experi- 

 ment. The wooden boxes in Nos. 1, 2, and 3, had been destroyed. 

 The temperature in No. 2 had been above 600, and in Nos. 1 and 3, 

 above 300, but not to 600, though probably not far below. 



The wooden box in No. 4 was as fresh as when put in. The ther- 

 mometer bursting at 150 was destroyed, but that bursting at 212 

 was sound. The heat had not attained to that of boiling water. It 

 will be borne in mind that No. 1 is the ordinary cement safe, No. 2 is 

 the ordinary plaster of Paris safe, No. 3 is the alum safe, and No. 4 

 the new safe. The first three were destroyed, while the temperature 

 in No. 4 was, at the utmost, entirely within the range of safety to the 

 books and papers. 



Conclusions. 1st. It is evident that the protection against fire is 

 mainly proportioned to the quantity of water the safe can give up to ~be 

 carried away as steam, and not to the non-conducting quality of its filling. 



2d. It is evident, further, that the protection against fire is not 

 simply as the quantity of water that may be present in the composition 

 for filling, but as the quantity of water that may be parted with unre- 

 strained by chemical affinity, or WATER AS SUCH. The more powerful 

 the chemical affinity resisting the escape of vapor, the more elevated 

 must be the temperature at which it will leave, while the capacity of 

 the escaping vapor to render heat latent or to absorb and carry it 

 away will remain unchanged. The same quantity of water in combi- 

 nation in alum is not so serviceable in keeping down the temperature 

 as when free. 



3d. It is evident, further, that while the water, in its uncombined 

 or natural state, must constitute a large part of the filling of a safe in 

 order to make its protection against fire in the highest degree avail- 

 able, this water must be held in solid form so as to give strength to the 

 safe ; and the safe must be so constructed as to prevent the water from 

 passing off by leakage or as vapor, to the injury of the books and pa- 

 pers, or to the lessening of the fire-proof qualities of the safe ; and yet 

 be so constructed as to allow, on the application of high heat, the most 

 free escape of vapor from those points to which the heat is applied, 

 without endangering the strength of the safe, or driving the vapor into 

 the interior chamber of the safe ; and withal so arranged as to permit 

 freezing, without injury to the safe or its contents. 



In a safe made in the light of the foregoing experiments, from 70 to 

 80 per cent, of the space appropriated to filling was occupied by 

 water, and yet was exposed for a day and two nights to a temperature 

 of zero without injury. 



On exposure to fire the water is resolved into vapor first at the outer 

 surface of the filling, and leaves the best non-conductor, according to 

 the results of foregoing experiments between the water which remains 

 and the heated metal of the exterior shell. At length, when all the 

 water has been driven out as vapor, there remains the non-conductor 

 of the whole thickness of the filling, to protect, as long as it may, the 

 contents of the case. 



