We may go one step further. If we take some sheets of perforated zinc or wire gauze, or even a large packet of pins, or paper bag full of iron filings, we shall find that all these bodies are practically opaque to the electric rays. Moreover, we can show that not only is the above law true for solids, but it holds good for liquids as well. I have provided here a number of flat glass bottles which are filled with various liquids, salt water, fresh water, solution of soda, paraffin oil, olive oil, turpentine and methylated spirits. If we test an empty glass bottle between the radiator and receiver, we can assure ourselves that the bottle itself is transparent to the electric radiation. If, then, we take the bottles containing the various liquids and test them one by one between the radiator and the receiver, we find that the bottles containing the paraffin oil, the olive oil, and the turpentine are transparent to the electric radiation, but that the bottles containing the salt water, the fresh water, the solution of soda, and the methylated spirits are all opaque. The oils and liquids similar to them are all good non-conductors, whereas water and various aqueous solutions are fairly good conductors of electricity, and hence these liquids, although they are all about equally transparent to light, behave very differently to electric radiation. As regards the electric ray, a bottle full of pure water is as opaque to the electric radiation we are here using as it would be to light if it were filled with black ink. Experiment shows that every object containing water, or which is wet, is exceedingly opaque to the electric radiation we are employing. Thus, for instance, if I take a dry duster folded in four, and hold it in the path of the electric ray, you see that it is quite transparent, and that the bell attached to the receiver rings as easily as if there were no duster there at all. If, however, we dip the duster in water, and then hold it between the radiator and receiver, we find that the wet duster is perfectly opaque. The human body consists largely of water which exists in the tissues, and hence it is not surprising to find that the hand or any part of the body placed between the radiator and receiver intercepts the electric ray. You see, if I hold my hand in front of the radiator, that nothing is able to escape from it, when sparks are made between the balls, which can affect the receiver. In the same way it can be shown by experiment that the human head is perfectly opaque—in fact, much more opaque than an equally thick block of wood; and this opacity to the electric ray is due in a veritable sense to the water in the brain. All dry animal tissues, such as leather, bone, gelatine, and flesh, if dry, are very transparent to electric radiation of the kind we are now using, but if these objects are made thoroughly wet, then they become intensely opaque. [Illustration: FIG. 76.—The reflection of an electric ray.]