We may admit that certain of their theories are rather ingenious; and what they say of the part played by the “shells,” for instance, or the “elementals,” in the spiritualistic phenomena, is worth about as much as our clumsy explanations of fluidic and supersensible bodies. Perhaps, or even no doubt, they are right when they insist that everything around us is full of living, sentient forms, of diverse and innumerous types, “as different from one another as a blade of grass and a tiger, or a tiger and a man,” which are incessantly brushing against us and through which we pass unawares. If all the religions have overpopulated the world with invisible beings, we have perhaps depopulated it too completely; and it is extremely possible that we shall find one day that the mistake was not on the side which one imagined. As Sir William Crookes well puts it in a remarkable passage: It is not improbable that other sentient beings have organs of sense which do not respond to some or any of the rays to which our eyes are sensitive, but are able to appreciate other vibrations to which we are blind. Such beings would practically be living in a different world to our own. Imagine, for instance, what idea we should form of surrounding objects were we endowed with eyes not sensitive to the ordinary rays of light but sensitive to the vibrations concerned in electric and magnetic phenomena. Glass and crystal would be among the most opaque of bodies. Metals would be more or less transparent, and a telegraph wire through the air would look like a long narrow hole drilled through an impervious solid body. A dynamo in active work would resemble a conflagration, whilst a permanent magnet would realise the dream of mediæval mystics and become an everlasting lamp with no expenditure of energy or consumption of fuel.