330 RADIANT . HEAT. 



determinations are much wanted of the degree to which the expan- 

 sion of the bulb influences the accuracy of air thermometers. The 

 improvement of mercurial thermometers so as to produce an instrument 

 of extreme sensibility to the minutest effects of heat, is an object the 

 attainment of which would probably be more important than that of 

 any other means for accomplishing the end in view. But other 

 methods, founded on good principles, should be diligently sought for 

 and tried; for example, it might be matter of inquiry whether we 

 could render available to this purpose the incipient melting or soft- 

 ening of some substances by a very slight increase of heat, or the 

 evaporation of volatile liquids. 



But it is more particularly desirable that the instrument of MM. 

 Nobili and Melloni should be tried, and a precise examination set on 

 foot of its real accuracy and the causes of error to whose influence it 

 may be liable. This is the more necessary from the very remarkable 

 character of many of their results, whilst the alleged sensibility of 

 the instrument, as they describe it, is such as almost to exceed belief. 



When we shall have succeeded in obtaining that prime requisite, 

 an unexceptionable measure of minute effects of radiant heat, we may 

 then proceed with some hopes of success to examine the points on 

 which there at present prevails so wide a discrepancy between differ- 

 ent experimenters. 



The polarization of heat is, perhaps, the question which, of all 

 others, requires the most extreme sensibility in our thermometer, or 

 rather thermoscope, in order to its satisfactory determination. It may 

 be tried, either directly, with the simple heat from non-luminous hot 

 bodies, or with luminous sources, with and without a glass screen, 

 comparing the total compound result with that due to the transmis- 

 sible part, or heating power of light alone, and thence deducing the 

 part due to simple heat. .The main difficulty is that of getting any 

 indication at all, after two reflections from plane surfaces. 



Another point which requires further investigation is the apparent 

 transmission of simple heat through very thin transparent screens, 

 but not through opaque. This should be examined in connexion with 

 the acute remark of MM. Nobili and Melloni, that a thin stratum of soot 

 may retain its low conducting power, and thus intercept the effect. 

 This, of itself, would form a subject for an accurate series of experi- 

 ments, viz: whether the ratios of the conducting powers of substances 

 remain the same for all thicknesses. 



The very nature of the transmissive and interceptive powers of 

 screens is little understood. Supposing simple heat transmitted with- 

 out diminution, how far is the mjjde of such transmission analogous 

 to that of light? what time is required for a body to commence ra- 

 diating heat after it has begun to acquire it? whether it acquires it 

 from a distant source instantaneously? how the heat distributes itself 

 upon or through a screen ? what is precisely the, effect of a coating 

 on one side of the screen in relation to the last question? upon what 

 the singular exceptions and anomalies pointed out by Melloni and 

 Nobili depend? whether any other such apparently anomalous c^ses 

 can be found? These are a few of the most obvious questions which 



