The secondary rays were found to be fairly homogeneous, for the intensity fell off according to an exponential law with the distance traversed. The value of the absorption constant λ was determined from the usual equation $$ \frac {I} {I₀} = e^{–λ d} $$, where _d_ is the thickness of the screen. The table given below shows the results obtained when thick plates of different substances of the same dimensions were placed in a definite position at _M_. The secondary radiation from fluids was obtained by a slight alteration of the experimental arrangements. Thirty milligrammes of radium bromide were used, and the results are expressed in terms of the number of scale divisions passed over per second by the gold-leaf. It will be noticed that the amount of secondary radiation follows in most cases the same order as the densities, and is greatest for mercury. The value of (secondary radiation)/density is not a constant, but varies considerably, being greatest for light substances. The absorption constant of the secondary rays from different radiators is not very different, with the exception of substances such as granite, brick, and cement, which give out secondary rays of nearly twice the penetrating power of other substances. β _and_ γ _rays_. Radiator Density Secondary Sec. Aluminium Radiation Rad. / ·085 cm. Density λ Mercury 13·6 147 10·8 Lead 11·4 141 12·4 18·5 Copper 8·8 79 9·0 20 Brass 8·4 81 9·6 21 Iron 7·8 75 9·6 20 (wrought) Tin 7·4 73 9·9 20·3 Zinc 7·0 79 11·3 Granite 2·7 54 20·0 12·4 Slate 2·6 53 20·4 12·1 Aluminium 2·6 42 16·1 24 Glass 2·5 44 17·6 24 Cement 2·4 47 19·6 13·5 Brick 2·2 49 22·3 13·0 Ebonite 1·1 32 29·1 26 Water 1·0 24 24·0 21 Ice ·92 26 28·2 Paraffin ·9 17 18·8 21 solid „ liquid ·85 16 18·8 Mahogany ·56 21·4 38·2 23 Paper ·4? 21·0 52 22 Millboard ·4? 19·4 48 20·5 Papier-mâché ... 21·9 Basswood ·36 20·7 57 22 Pine ·35 21·8 62 21 X ray screen 75·2 23·6