58 



HEAT. 



Mr. Watt, in the course of his ex- 

 periments on watery vapours, was led 

 to expect that a temperature of 70 

 would be sufficient for the distillation 

 of water in a vessel exhausted of air, 

 the refrigerator being immersed in 

 snow. The experiment was tried by 

 him and succeeded ; he used a small 

 still, which was about half filled with 

 water and then securely joined to a 

 receiver, having an aperture below. 

 As soon as the water in the still was 

 made to boil, the vapour filled the 

 receiver and expelled the air at the open 

 aperture, which was then closed with a 

 plug. The still was then cooled by 

 being set on ice, which occasioned the 

 condensation of steam in the receiver. 

 On the application of a lamp, steam 

 was again produced ; and, on im- 

 mersing the receiver in cold water, dis- 

 tillation proceeded at a low tempera- 

 ture. The noise of boiling was dis- 

 tinctly heard in the still, although the 

 top of it scarcely seemed warm to the 

 hand. Mr. Watt found from this and 

 other experiments, conducted with 

 greater care, that, although distillation 

 may be effected with very little heat in 

 vacua, no advantage in regard to the 

 saving of fuel can be obtained, as the 

 latent heat of the steam is increased in 

 proportion to the diminution of sensi- 

 ble heat. 



Distillation in vacua has, notwith- 

 standing, been adopted in several cases, 

 with great success, where excellence in 

 the products of distillation is the prin- 

 cipal object. 



This process has been adopted by 

 Mr. Henry Tritton, who uses a still 

 nearly of the common form, which is 

 immersed in hot water, instead of being 

 placed in close contact with fire. The 

 pipe proceeding from the upper part of 

 the still, passes, in the usual way, 

 through a vessel containing a large 

 quantity of cold water, and then com- 

 municates with a capacious vessel 

 which is capable of being exhausted of 

 air by an exhausting syringe, or air 

 pump, attached to it. When the water 

 surrounding the still is heated, and the 

 air withdrawn, distillation proceeds ra- 

 pidly, at a very low temperature. 



The same process has been adopted, 

 with great advantage, in the distilla- 

 tion of vinegar, at the extensive and 

 admirably-managed vinegar manufac- 

 tor of Messrs. Charles Pott and Co., 

 on the Surrey-side of Southwark bridge. 



Vinegar, subjected to distillation, is 

 freed from its usual colour, and ren- 

 dered perfectly pellucid. It was found 

 impossible, however, to distil it in the 

 usual way without imparting a disagree- 

 able burned kind of flavour ; this is 

 entirely avoided by distillation in vacua, 

 as the heat of the vapour raised is 

 only 130 of Fahrenheit's scale. 



The process of distillation by which a 

 gaseous bodyis set free from one state of 

 combination, for the purpose of making 

 it enter into another, is of a different 

 kind, and requires different manage- 

 ment. This may be illustrated by a 

 brief description of the process for ob- 

 taining liquid muriatic acid, upon a 

 small scale. Five parts of strong sul- 

 phuric acid having been added to six 

 parts of decrepitated sea- salt, in a 

 tubulated glass retort, muriatic acid 

 gas is set free, and is made to pass 

 through water, contained in a series of 

 glass vessels, called Woolfe's bottles, 

 which are so contrived, that the gas 

 may be subjected to a degree of pres- 

 sure, for the purpose of facilitating its 

 union with the water. Safety tubes are 

 applied to prevent danger from the 

 elastic pressure of the gas, and also 

 from its sudden condensation. The 

 process being continued until the water 

 is saturated with the gas, it becomes 

 liquid muriatic acid. 



Respecting the cause of liquefaction, 

 vaporization, and other phenomena 

 which have been described, there have 

 been differences of opinion. Dr. Black 

 believed that those changes are occa- 

 sioned by the absorption of heat, which 

 remains combined with bodies that 

 have changed from the solid to the 

 fluid state, until they resume the solid 

 state again ; and with bodies which 

 have changed from the fluid to the 

 aeriform state, until they resume the 

 fluid state again. Although Dr. Black 

 did not express himself clearly upon 

 this point, there is reason to believe 

 that he considered the caloric, while in 

 a latent or concealed state, to be com- 

 bined with the bodies, in a way which 

 bears a strong analogy to chemical 

 combination. 



Dr. Irvine maintained that when 

 heat is absorbed by bodies, in conse- 

 quence of a change in their state, it 

 does not enter into combination with 

 them, but exists in them in the same 

 state as the other caloric which they 

 contain ; the increased quantity being 



