224 PHILOSOPHICAL TRANSACTIONS. [aNNO 1738. 



creams, in places where salt is dear, as in France, where it is sold for lO sols 

 a pound. 1st, Because in France this pot-ash is sold only for 2-i- sols a pound. 

 2dly, Because, not freezing so fast, it does not spoil the creams by reducing 

 them to icicles. 3dly, Because ice-creams made this way, will keep longer in 

 a condition fit to serve at table. 



11. Sugar J 4 deg. 



12. Alum IJL 



13. Salt of glass 10 



14. Sal ammoniac 12f 



] 5. Quick-lime 1^ 



1 6. Glaubers salt 2 



17. The cold of ice may still be considerably increased by a mixture of spirit 

 of wine ; about a drinking-glass full of spirit of wine to a pound of beaten ice. 



18. The cold of ice will not increase, unless the ice melts. 

 Experiments. — Put into one vessel 4 ounces of ice, beaten very small, and 



into another vessel 2 ounces of sea-salt ; set the two vessels in a mixture of 

 ice and salt, which is to be renewed still, till by means of the thermometer 

 you find, that the salt and the ice of the first two vessels have acquired each of 

 them 10 or 12 degrees of cold; then mix the salt with ice, and this mixture 

 will not increase the degree of cold that the ingredients had acquired, because 

 the mixtcire does not melt. But if, instead of salt, you mix with the ice 

 spirit of nitre, cooled to the same degree as the ice, as this last is liquid, it 

 will melt the ice, and considerably increase its cold. 



19. Salt mixed with water, increases its cold. 



20. Of all salts, sal ammoniac gives the greatest degree of cold;* so that if 

 that salt has been cooled in ice, and then one part of it be thrown into 2 parts 

 of water, cooled to the same degree in ice, that water will become colder than 

 ice, and will freeze other water thrown into it in a small quantity. 



This last observation may be applied to the cooling of liquors where no ice 

 is to be had ; for there is hardly any place, but what has wells : now the water 

 of a well moderately deep, wants about 8 or 10 degrees of the cold of ice; 

 and sal ammoniac being cooled beforehand in the well, will, by mixing with 

 some of the water of that well, come very near to the cold of ice. 



An Observation of the Magnetic Needle being so affected by Cold, that it 

 would not traverse; by Capt. Christopher Middleton, F.R.S. N°449, p. 310. 



In the Philos. Trans. N°418, Capt. M. mentioned a strange phenomenon 

 relating to the sea-compass, which he had frequently observed, when among 



* Mr. "Walker of Oxford has shown that there are other salts which are preferable to sal ammoniac 

 for producing artificial cold, and particularly muriate of lime. See Phil. Trans, for 1795 and 1801. 



