24 REPORT — 1841. 



will lead to what we seek. They were not originally conducted with refer- 

 ence to the state of things assumed to exist in theory, and are, in conse- 

 quence, of less value when allowance is made for the difference between what 

 they express and what theory requires. Now we do not deny that difficulties 

 do attend the experimental examination of this subject, when it is intended 

 to make everything correspond with the state supposed in theory. The 

 chief and greatest of these we conceive to arise from the presence of the air. 

 MM. Dulong and Petit have shown that the quantity of beat carried off by 

 the air is not only very large, but is governed by a law very different from 

 that of ordinary radiation. Means have therefore to be devised for removing 

 this cause of error ; but we are far from thinking that the difficulty of effect- 

 ing this amounts to an impossibility. If MM. Dulong and Petit could suc- 

 ceed in determining the rates of cooling of a body hi vacuo, we cannot see 

 why others should not succeed in observing the stationary temperature at one 

 point, at least, of a body which radiates in vacuo. This leads us to the sug- 

 gestions with which we shall conclude the present Report. We shall offer 

 two: 1st, as to the most important experiments; 2ndly, as to the mode by 

 which they may be conducted. 



It is perhaps chargeable against the theoretical writers on this branch of 

 physics, and especially against M. Poisson, that they have not presented their 

 results in a form sufficiently tangible to direct or suggest the application of 

 experiment to them. It is much to be regretted that no attempt has been 

 made to obviate this. With the view of remedying the state of things to a 

 certain extent, we have exhibited in their most simple forms some of the 

 more obvious conclusions to which the different theories lead. No doubt 

 much might be done in this way, but, until called for by the entry of expe- 

 rimenters on the field, a large and varied collection of formulae would serve 

 no useful purpose. One class of experiments alone appears amply to suffice 

 for our present purpose. The object being, to discover a law of conduction, 

 it will be best attained by the selection of circumstances in which radiation 

 either plays no part at all, or in which its effect is very simple and readily 

 eliminated. The former condition exists in the problem which is solved by 

 formulas 1, 15 and 16. By selecting a substance of small conducting power, 

 such as marble, and coating the block with a substance which will radiate 

 very slowly, this experiment may be made on a block of no very great di- 

 mensions. For many reasons this experiment is well worth trying. It will 

 probably distinguish at once between the three theories. It will certainly 

 offer strong reasons for rejecting either the third or the fourth. Of course 

 it will hardly serve to establish directly either of them. To effect this, we 

 would point out another most important experiment, — The determination of 

 tlie state of temperature at one extremity of a bar wJiich is heated at the other 

 extremity. This experiment should be made on a variety of bars, of different 

 conducting powers and of different lengths. With a set of careful experi- 

 ments of this nature, we believe we could pronounce, without fear, the true 

 law of conduction. Nor do we tliink the difficulties attendant on the con- 

 duct of the experiments to be at all insuperable. The greatest obstacle is, 

 no doubt, the expense of apparatus ; but where we find expense overruled in 

 the prosecution of experimental researches into less important and certainly 

 not more interesting branches of physics, Avhere theory has hardly opened a 

 field for speculation, and where curiosity alone prompts the inquiry, it mus't 

 excite our surprise that so little has been done in this case, which presents 

 analytical developments of great beauty, and, independently of its close con- 

 nexion with the favourite theories of light and the discoveries of chemistry, 

 deserves to rank high amongst the physico-mathematical sciences. But, 



