202 



Mr. Daniell's Further Experiments 



undergo to their fusing points, it is clear that, had their ex- 

 pansion been equal for equal increments, we might have de- 

 termined the true temperature of their melting points from 

 these data. As it is, even, knowing something of the limits 

 of error introduced into such a calculation by the increased 

 rate of expansion at the upper part of the scale, and the di- 

 rection in which it ought to affect the result, we may draw 

 some important inferences with regard to the correctness of 

 the determinations derived from other means. The follow- 

 ing Table exhibits the results of such a calculation, compared 

 with the melting points previously determined. 



Table XV. 



Fusing Points of Metals derived Ji-an their Expansions to 212° 

 and 662° supposed equable. 



Now by these results, the accuracy of the pyrometer may, 

 again, be placed beyond doubt, in a manner which was per- 

 fectly unforeseen at the time of instituting the experiments. 



In the first place we have two metals, tin and lead, whose 

 melting points being within the temperature of boiling mer- 

 cury, have been accurately determined by the common ther- 

 mometer. Upon calculating the same points from their several 

 expansions to boiling water, measured by the pyrometer, upon 

 the supposition that they maintain the same rate to their points 

 of fusion, the temperature of the first comes out 29°, and of 

 the second 58°, higher : that is to say, the rate of expansion of 

 these two metals increases with the increase of temperature, as 

 has been found to be the case with platinum, iron and copper, 

 by the experiments of MM. Dulong and Petit. It is worthy 

 of remark, that this increased rate in tin is equivalent to 29" 

 in about 200°, and in lead to 58° in about 400°, above the boil- 

 ing point of water. These results therefore indicate a very 

 close agreement between the thermometer and pyrometer. 



2ndly. The melting point of the next metal, zinc, is one of 

 those which has been determined by immersion of the pyro- 

 meter into it, when it was in the act of fusion. Its tempera- 

 ture, so determined, falls short of the same point, calculated 



