Chemical Science. ' ' 221 



in the place. The temperature of the air was ascertained by a 

 sensible thermometer, and it was found easy to make it nearly 

 uniform, for the whole of an experiment. Each experiment con- 

 tinued six hours, and it was only after two or three hours, that all 

 the thermometers became stationary. The thermometer nearest to 

 the source of heat soon acquires the temperature at which it is 

 to be retained stationary, and then the heat is managed so that 

 it shall not rise or fail by that instrument, until the experiment is 

 finished. The following is the form of an experiment. 



Bar of Copper. Temperature of the Air 17°. 08 C. 



Vrppss ahovp tho Quotient of tho sum of 



Thermometer. Temperature. tpmnofair two excesses, divided by 



o temp^otair. the intermediate excess. 



1st 83.44 66.36 



2nd 63.36 46.28 2.14 



3rd 49.70 32.62 2.15 



4th 41.40 24.32 2.11 



5th 35.71 18.63 2.17 



6th 33.26 16.18 

 It is admitted as having been demonstrated, that the conducti- 



bility is proportional to ; dC being obtained by the equation 



X -\- — = 7 ; in which q is the coefficient of the sum of the ex^ 



X 



cess, by the intermediate excess : and according to the results 

 obtained with good conductors, as gold, silver, platina, copper, iron 

 and zinc, these satisfy the experimental series which is indicated 

 by calculation. The same is not the case with bad conductors. 



Wood conducts so feebly, that a bar 21 millimetres does not be- 

 come sensibly heated a few centimetres from one of its extremities 

 so far raised in temperature as to carbonize the substance. — An^ 

 iiales de Chimie, xxxvi. 422. 



3. On the relation ofW^ater to hot polished Surfaces. — Tlie tran- 

 quil state of a drop of water in a very hot silver teaspoon, or 

 metallic capsule, with the comparative lengthened period of its 

 evaporation, are facts well known, and are usually explained by ad- 

 mitting the intervention of a film of vapour which prevents the con- 

 tact of the water and the metal, and so interferes with the trans- 

 mission of heat. Mr. Perkins thinks he has proved that other 

 causes are importantly active ; but without referring to the opinions 

 on this point, I have thought it may be interesting to point out 

 another form of the experiment which 1 have often witnessed. A 

 large trough of water being placed under the fire bars of a power- 

 ful furnace, the water soon becomes heated by the fall of ashes into 

 it, and the communication of heat both by radiation and condensation. 

 With the ashes fall numerous small globules of slag highly lifated, 

 and these will frequently remain upon the surface of the water, 

 slightly depressing it at the place, and will float quietly about for 



