194 Transactions of the Society. 



These are of glass and are immersed iu the water. In the upper rims 

 of these vessels, which are outside the copper container, C C C are loosely 

 fittino- cork tops, in the centre of which delicate thermometers are fixed, 

 so as to have their bulbs plunged into the putrefactive fluid. D D are 

 two thermometers of a similar kind placed in the water of the copper 

 vessel to register its temperature. E E E E E are copper tubes fixed 

 across the copper vessel, some distance from the bottom, to form a firm 

 support for the glass tube " gridiron " F F ; this is continuous with the 

 tube GG, and is filled, from somewhere near H, throughout all its 

 length, with about ten pounds of mercury. The bulb J is also full of 

 mercury, and K is a steel screw plunger, which by being screwed in or 

 out can raise or lower the column of mercury above it. L is an ivory 

 index of the height of the mercury. M is a chamber larger than the 

 general tube G, and has a rectangular arm N ; into M is fixed by a 

 cork, air-tight, a smaller tube ; this is in direct communication with 

 the gasometer. In the bottom of this tube is fixed a tube of platinum, 

 which " wets " with mercury, making a sort of contact, but not an 

 amalgam. 



From the tube N proceeds all the gas that supplies the burner P. 

 The gas therefore, coming in at 0, finds its way out between the 

 bottom of the tube 0, and the top of the mercury, into the tube H, and 

 so to the burner. 



When the right point of temperature is attained, the steel plunger K 

 is screwed in until the mercury is close to contact with the platinum ; 

 and by about an hour or so of watching and manipulation the tempera- 

 ture is made static at the point required, for if the heat become slightly 

 higher than it should be, the mercury, by touching the platinum, stops 

 the supply of gas ; and the gas would go out, but that a very small hole 

 is pierced in the platinum of the tube opposite H, which, when the 

 platinum and mercury are in contact, sends enough gas to the burners to 

 keep up a flame in each, of more or less diminished intensity as the 

 nature of the case requires. Hence, with a smaller heating power in 

 the burners, the temperature falls very slightly ; and so does the column 

 of mercury: by this means the amount of gas sent to the burner is 

 increased, the heat is again raised a minute fraction, and thus, within 

 a quarter of a degree Fahr., the fluids may be kept at a fixed tempera- 

 ture, which is none the less variable at will. 



Two essentials are involved in the delicate working of this instru- 

 ment ; the first, especially with the lower grades of temperature, is a 

 room which shall be of a constantly even temperature ; and second, gas 

 for consumption at an unchanging pressure. 



This latter is now so comparatively easy of accomphshment, by 

 several useful instruments, such as Moitessier's gas pressure regulator, 

 that it is needless to burden you with details of the less elegant and more 

 laborious method which I employed in the earlier days. They were, 

 however, quite efficient, and I have now two thermostats at work on this 

 matter in which the variation is less than a sixth of a degree Fahr. in 

 twenty-four hours. 



For greater accuracy in action it is well to have several small Bunsen 

 burners grouped, each witli a separate stop tap, and the whole capable of 



