These two problems are often wrongly confused.

The gap between the two classes of bodies may be filled by certain substances with intermediate properties, such as pasty bodies and bodies liquid but still crystallized, because they have not yet completely lost their peculiar structure.

Yet the transition is not necessarily established in a continuous fashion when we are dealing with the passage of one and the same determinate substance from the liquid to the solid form.

We conceive that this change may take place by insensible degrees in the case of an amorphous body.

But it seems hardly possible to consider the case of a crystal, in which molecular movements must be essentially regular, as a natural sequence to the case of the liquid where we are, on the contrary, in presence of an extremely disordered state of movement.
M.Tamman has demonstrated that amorphous solids may very well, in fact, be regarded as superposed liquids endowed with very great viscosity.