If, on the other hand, we violently compress the cell, if we pass a very strong electric shock through it, or a strong continuous current, or expose it to a temperature much above 45° C., or to the action of certain chemical substances, such as strong acids or alkalies, or alcohol or corrosive sublimate, we find that all these vital processes are arrested once and for all; henceforward the cell is on a par with any not-living substance. Such a change is called "DEATH," and the "capacity for death" is one of the most marked characters of living beings. This change is associated with changes in the mechanical and optical properties of the protoplasm, which loses its viscidity and becomes opaque, having undergone a process of _de_-solution; for the water it contained is now held only mechanically in the interstices of a network, or in cavities of a honeycomb (as we have noted above, p. 5), while the solid forming the residuum has a refractive index of a little over 1.6. Therefore, it only regains its full transparency when the water is replaced by a liquid of high refractive index, such as an essential oil or phenol. A similar change may be effected by pouring white of egg into boiling water or absolute alcohol, and is attended with the same optical results. The study of the behaviour of coagulable colloids has been recently studied by Fischer and by Hardy, and has been of the utmost service in our interpretation of the microscopical appearances shown in biological specimens under the microscope.[13] {12}The death of the living being finds a certain analogy in the breaking up or the wearing out of a piece of machinery; but in no piece of machinery do we find the varied irritabilities, all conducive to the well-being of the organism (under ordinary conditions), or the so-called "automatic processes"[14] that enable the living being to go through its characteristic functions, to grow, and as we shall see, even to turn conditions unfavourable for active life and growth to the ultimate weal of the species (see p. 32). At the same time, we fully recognise that for supplies of matter and energy the organism, like the machine, depends absolutely on sources from without. The debtor and creditor sheet, in respect of matter and energy, can be proved to balance between the outside world and Higher Organisms with the utmost accuracy that our instruments can attain; and we _infer_ that this holds for the Lower Organisms also. Many of the changes within the organism can be expressed in terms of chemistry and physics; but it is far more impossible to state them _all_ in such terms than it would be to describe a polyphase electrical installation in terms of dynamics and hydraulics. And so far at least we are justified in speaking of "vital forces."