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REPORT — 1859. 



To illustrate the principles of the saturation gauge, suppose two globes, A and B, 

 fig. 1, connected by a bent tube containing mer- 

 cury at a b, and placed in a bath in which they can 

 he raised to any required temperature. Suppose a 

 Torricellian vacuum to have been created in each 

 globe, and twenty grains of water to have been 

 added to A, and thirty or forty grains to B. Now, 

 suppose the temperature to be slowly and uniformly 

 raised around these globes ; the water in each will 

 go on evaporating at each temperature, being filled 

 with steam of a density corresponding to that tem- 

 perature, and the density being greater as the tem- 

 perature increases. At last a point will be reached 

 at which the whole of the water in globe a will be 

 converted into steam, and at this point the mer- 

 cury column will rise at a and sink at b ; this is the saturation test, and the cause of 

 its action will be easily seen. So long as vaporization went on in both A and B, and 

 the temperature was maintained uniform, each globe would contain steam of the same 

 pressure, and the columns of mercury, a and b, would remain at the same level. But 

 so soon as the water in A had vaporized, and the steam began to superheat, the press- 

 ure on a would cease to remain uniform with the pressure on b, and the mercury 

 column would at once fall, and thus indicate the difference. The instantaneous 

 change of the position of the mercury is the indication of the point at which the tem- 

 perature in the bath corresponds with the saturation point of the steam in A. 



To show the delicacy of this test, I may instance, 

 that at 290° Fahrenheit, the mercury column would 

 rise nearly two inches for every degree of temperature 

 above the saturation point, as the increase of pressure 

 arising from vaporization is about twelve times that 

 arising from expansion in superheating at that point, 

 and a similar difference exists at other temperatures. 



The arrangement of the apparatus, as employed 

 for experiment, varies according to the pressure and 

 other circumstances of its use. Fig. 2 represents one 

 of the arrangements which has been employed with 

 success. It consists of a glass globe A of about 

 seventy cubic inches capacity, in which is placed, after 

 a Torricellian vacuum has been formed, the weighed 

 globule of water; this is surrounded by a copper 

 boiler B B, prolonged by a stout glass tube C C, en- 

 closing the globe stem. This copper boiler forms the 

 water- and steam-bath through which the globe is 

 heated, and in fact corresponds to the second globe B 

 in the former figure. The fluctuating mercury column, 

 or saturation gauge, is placed at the bottom of the tube 

 C C, and the saturation point is indicated by the rise of 

 the inner mercury column b, and the fall at the same 

 time of the outer mercury column c. As soon as the 

 whole of the water in the globe A is evaporated, there is 

 an instantaneous rise of the inner mercury column to re- 

 store the balance of pressure, and that progressively 

 with the rise of temperature. 



As an auxiliary apparatus the boiler is provided with 

 gas-jets, E, to heat it, and with an open oil bath G to 

 retain the glass tubes at the same temperature as the 

 boiler, and this oil-bath is placed on a sand-bath, and 

 also heated with gas. A thermometer D registers the 

 temperature, and a pressure gauge F the pressure of 

 the steam; and a blow-off cock H serves to reduce the 

 temperature when necessary. A number of results 



