512 



MISCELLANEOUS GARDEN STRUCTURES. 



retarded, and the action of the warmer 

 current upon the body of ice diminished." 



We believe the practice of throwing 

 salt on the ice, during the process of 

 filling the house, is not only useless but 

 injurious. No doubt the practice owes 

 its origin to the well-known experiment 

 of producing an intense degree of cold 

 by mixing salt with either snow or ice — 

 a practice still continued by confectioners 

 in preparing ices for the table. 



On this subject we find the following 

 reasoning by Mr Corbett, a very intelli- 

 gent gentleman, in the work last quoted : 

 " In a temperature of 40° Fahr., a certain 

 weight of ice takes ten hours to become 

 fluid : in the same time, and with the 

 same source of heat, 140 times the weight 

 of water would be raised 1° in tempera- 

 ture ; therefore 140 times the quantity of 

 heat is required to convert ice into water, 

 as to raise the same weight of water 1°. 

 Allowing the air and ice, w T hen filling the 

 ice-house, to be 32°, if salt be thrown upon 

 the ice, a part of it immediately melts ; 

 and all that which melts, having no time 

 to draw upon the atmosphere for its source 

 of heat, (viz. 140°, which is immediately 

 destroyed in the act of melting,) becomes 

 in the water latent. Thus melted water 

 and ice, being suddenly reduced in tem- 

 perature, will lower the thermometer from 

 32° to zero. If only a small quantity of 

 salt is used, the surrounding ice, which 

 has not been reduced in temperature by 

 melting with salt, will gradually give out 

 some of its sensible heat, and will soon 

 become equalised with that in immediate 

 contact with the melting water ; but the 

 latter, having partly disseminated itself 

 through the whole mass, becomes partially 

 saline, and its freezing-point will be 

 lowered in proportion to the quantity of 

 salt used. After these sudden changes 

 have taken place, therefore, the difference 

 between the salted and unsalted ice-house 

 will be, that the latter is filled with ice at 

 the temperature of 32°, and whose freez- 

 ing-point will remain 32°, while every 

 grain of ice melted by transmitted heat 

 will give the protection of 1° of cold to 

 140° grains of ice surrounding it; whereas 

 the former will also be filled with ice 

 exactly at its freezing-point, which may 

 perhaps be 30°, and every grain of such 

 ice, when melted by transmitted heat, 

 will also give 1° of protection to 140 



grains of the unsalted ice, or will counter- 

 act the transmitted and continually trans- 

 mitting heat to that extent. But with 

 what velocity will that heat be transmit- 

 ted? Exactly in proportion to the dif- 

 ference in temperature between the stored 

 ice and the medium which surrounds it. 

 If we allow the surrounding medium to 

 be 34°, then the velocity of the transmit- 

 ted heat from that medium to the ice will 

 be twice as much in the salted house — 

 viz. from 34 to 30° — the difference being 

 4°, as in the other, which would only be 

 from 34° to 32°, the difference being only 

 2° ; consequently the ice would keep twice 

 as long in the one as in the other." Mr 

 Corbett observes that he does not pretend 

 to say " that these presumed temperatures 

 are such as really occur, but they must of 

 necessity approach it ; and if so, the ar- 

 gument remains the same." 



Before filling the ice-house, every means 

 should be taken to dry the walls as tho- 

 roughly as possible. For this purpose the 

 doors should be kept open in dry windy 

 days, to admit of the fullest amount of 

 ventilation; and that this may be the 

 more completely accomplished, a venti- 

 lator or chimney should be built on the 

 highest part of the roof of such houses as 

 are so constructed as to be filled from the 

 doors only. A bushel or two of unslaked 

 lime may be with great advantage em- 

 ployed, by being placed within the ice- 

 well, as it is an absorber of water to the 

 extent of 5 gallons to every bushel. 



The production of ice by artificial 

 means, from what has already been stated, 

 is not a modern invention, although the 

 most convenient mode of bringing about 

 that process is so. Those who wish to 

 manufacture their own ice should study 

 the principles laid down by Mr Masters 

 in " Treatise on the Production of Ice and 

 Artificial Cold," from which the following 

 extract and table are taken : "The cooling- 

 power of different substances undergoing 

 solution may be represented by various 

 relative quantities, according to their 

 power of absorbing heat; and, from a 

 long series of experiments, I have at 

 length prepared a compound capable of 

 accomplishing this effect to a much greater 

 degree than any hitherto known mixture. 

 Still, however, let it be understood, I 

 do not pronounce it specific against any 

 uncommon positions. Let the cooling 



