VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 503 



in hot climates especially, where the inhabitants scarcely know by the sense of 

 feeling winter from summer, it may not be amiss to hint at the easiest and most 

 economical method of using them. For most intentions perhaps, the following 

 cheap one may be sufficient: of strong vitriolic acid, diluted with an equal weight 

 of water, and cooled to the temperature of the air, any quantity; add to this an 

 equal weight of vitriolated natron in powder: this is the proportion when the 

 temperature set out with is + 50°, and will sink the thermometer to 5^; if 

 higher, the quantity of salt must be proportionably increased. The obvious and 

 best method of finding the necessary quantity of any salt to produce the greatest 

 effect, by solution in any liquid, at any given temperature, is by adding it gra- 

 dually until the thermometer ceases to sink, stirring the mixture all the while. 



If a more intense cold be required, double aqua fortis, as it is called, 

 may be used; vitriolated natron, in powder, added to this, produces very nearly 

 as much cold as when added to the diluted nitrous acid: it requires a rather 

 larger quantity of the salt, at the temperature of-|- 50", about 3 parts of the 

 salt to 1 parts of the acid: it will sink the thermometer from that temperature 

 nearly to O, and the consequence of more salt being required is, its retaining 

 the cold rather longer. This mixture has one great recommendation, a saving 

 of time and trouble. A little water in a phial, immersed in a small tea cup of 

 this mixture, will be soon frozen in summer; and if the salt be added in crystals 

 impounded to double aqua fortis, even at a warm temperature, the cold produced 

 will be sufficient to freeze water or creams; but if diluted with -f its weight of 

 water, and cooled, it is about equal to the diluted nitrous acid above-mentioned, 

 and requires the same proportion of the salt. A mixture of vitriolated natron 

 and diluted nitrous acid sunk the thermometer from + 70°, temperature of air 

 and ingredients, to -f- 10°. The cold in any of these mixtures may be kept up 

 a long time by occasional additions of the ingredients in the proportions men- 

 tioned. A chemist would make the same materials serve his purpose repeatedly. 



Equal parts of muriated ammonia and nitrated kali in powder make a cheap and 

 convenient composition for producing cold by solution in water; it will, by the 

 following management, freeze water or creams at Midsummer. June 12th, 

 1787, a very hot day, Mr. W. poured 4 oz. wine measure, of pump water, at 

 the temperature of 50° (it is well known that water at springs retains nearly the 

 same temperature winter and summer, viz. about 50°, to which temperature the 

 water may be reduced during the warmest weather, by pumping off^ some first) 

 on 3 oz. Avoirdupois weight, of the above powder (previously cooled bv im- 

 mersing the vessel containing it in other water at 50°,) and after stirring the 

 mixture its temperature was 14°; some water contained in a small phial, im- 

 mersed in this mixture, was consequently soon frozen. This solution was a^ 

 terwards evaporated to dryness, in an earthen vessel, reduced to powder, and 



