316 Daniell on a New Pyrometer. 



brass ferule and upper part of the tube, and kept moist with 

 water to guard against too great an accumulation of heat in 

 that part. I have not, however, myself had occasion to make 

 use of this expedient, although I have kept the instrument for 

 half an hour at a time at the temperature of fused iron. The 

 black-lead is so very bad a conductor of heat, that its trans- 

 mission is very slow from one end to the other. 



The value of the degrees of each instrument must be taken 

 for itself, and this is very easily done by means of the apparatus 

 before described. The boiling of mercury furnishes an ad- 

 mirable fixed point for this purpose. The number of degrees 

 equivalent to 656° of Fahrenheit should be marked upon the 

 scale. 



The motion of the index, as before stated, is very gradual, 

 and it stops directly the intensity of the fire ceases to increase. 

 The difference of a greater or less draught of air in a furnace is 

 instantly denoted by it. In the furnace which I have been in 

 the habit of employing, the opening of a small door instantly 

 increased the advance of the needle, and the closing it as sud- 

 denly checked it. When properly managed in cooling, the 

 needle never failed to return within two or three degrees of the 

 commencement of the scale ; but to ensure this effect, it is 

 necessary that the instrument should be cooled very gradually 

 in the furnace to which it has been applied, or that it should be 

 removed suddenly and at once into the cold air. If it be with- 

 drawn gradually, the partial cooling is apt to produce a slight 

 alteration in the form of the tube, the effect of unequal con- 

 traction. From this cause I have seen the index not return 

 within twenty or thirty degrees of the point from whence it 

 set out. This small deviation in the form of the tube when 

 it occurs is not of any serious practical consequence as the 

 index may always be set at the beginning of an experiment, to 

 the commencement of the scale, or to the temperature of the 

 air, without in any way aftecting the accuracy of the subsequent 

 observations. 



I shall conclude by recording the results of some experiments 

 which I have tried upon the fusing point of some of the metals. 



