184 L EGTUEES TO SCIENCE TEA CHEES. 



by Seebeck that if either end were heated more than the 

 other, a current would pass through this continuous series 

 of bars and through a wire which might be connected with 

 the extremity of the series and with a galvanometer, so 

 that all that was necessary in order to measure the increase 

 of temperature in one side over the other was to measure 

 the intensity of the current of electricity passing through 

 the wire. Here is another thermopile, and we shall see 

 presently, as soon as the beam falls on the surface, a 

 current of electricity will pass through it and be indicated 

 by the motion of the galvanometer. Employing this instru- 

 ment, Melloni was led to other important results. The 

 most important perhaps was his grand discovery that rock- 

 salt is a substance which is apparently absolutely perme- 

 able to heat. He believed there was no loss of heat by 

 absorption when radiant heat was passing through rock- 

 salt, but it is better to say that there was no absorption 

 perceptible with the instrument that he employed. Un- 

 fortunately rock-salt is a substance that deteriorates very 

 much by exposure to a moist atmosphere, and consequently 

 it cannot be employed in laboratories except under peculiar 

 circumstances, and it has to be preserved in a very dry 

 atmosphere. I have here a lens, a prism, and a plate of 

 rock-salt, which are exhibited in the Loan Exhibition by 

 Stegg, of Hamburg ; and in order to preserve them they 

 are surrounded by pieces of glass. Almost all the great 

 researches in radiant heat have been made by the aid of 

 rock-salt apparatus. Melloni measured the amount of heat 

 which is found in different parts of the spectrum, and also 

 contained the method which was employed by Delaroche 

 of sifting heat. "We find his results included in a consider- 

 able-sized volume called La Thermochrose, or Heat Colora- 

 tion, we may call it ; and he used flames and sources of 

 heat of different kinds. He found different results from 

 different kinds of heat. Thus when he employed a flame 

 sent through rock-salt he found only 92 per cent, got 

 through, some being reflected from the surface. "When he 

 employed plate-glass only 39 per cent, got through, and 

 with alum only 9 per cent. Then when he employed other 

 sources of light he got the heat proportionately, as shown 

 in this table : 



