Resonance Experiments with the Longest Heat- Waves. 441 



absolutely opaque to the heat-waves, for we know that the 

 reflecting power of a metal is practically 100 per cent, for all 

 waves longer than about 10 /x. The same thing is true of 

 carbon deposits. When studying the transmission of the 

 waves through various substances in collaboration with 

 Rubens, it was found that a deposit of smoke on a quartz 

 plate, so dense that the Welsbach mantle was invisible 

 through it, transmitted over 95 per cent, of the heat radia- 

 tion. It seemed of interest to ascertain the relation existing 

 between the transparency of a layer of metallic particles, and 

 the size of the particles, in other words, to carry out experi- 

 ments analogous to those on the action of tinfoil strips of 

 various size (mounted on glass) on electromagnetic waves. 

 Various methods may be used for the preparation of the 

 resonator plates. We may deposit a film of the metal on a 

 quartz plate and then rule it into small squares or rectangles 

 with a diamond point, on a dividing-engine. This method 

 was tried over fifteen years ago by Rubens and Nichols ; 

 much shorter heat-waves were then available (12 /a), and 

 though the experiments appeared to indicate electromagnetic 

 resonance, they were not as satisfying as was to be desired. 



Another method is to prepare metal particles of uniform 

 size and deposit these over the quartz surface. 



This was the method first adopted in the present case. I 

 first tried blowing a fusible alloy into a fine spray with an 

 atomizer (the method used by Professor Millikan and his 

 colleagues). 



Very perfect spherical droplets can be obtained in this way, 

 but difficulties were found in sorting them out into groups of 

 uniform size of particles, and in depositing them (after 

 sorting) with any regularity of distribution. It then occurred 

 to me to condense a metallic vapour on the quartz plates in 

 the form of a " dew." This method worked admirably. A 

 small quantity of mercury was heated in a beaker, and the 

 quartz plate suspended in a horizontal position about 2 cm. 

 above the surface of the metal. Very regular deposits were 

 obtained in this way, the size of the globules varying with 

 the duration of the exposure of the plate to the metallic 

 fumes. 



Photographs of the deposits taken with a microscope are 

 reproduced on PL III. figs. 1 to 6 inclusive. A photograph 

 of a " stage micrometer " scale is reproduced immediately 

 below the figures. One division on this scale corresponds to 

 0*01 mm. As is apparent from the photographs, the diameters 

 of the drops of a given deposit are fairly constant and their 

 distribution is pretty regular. 



