Another material which also seems to have the power of combining with a number of other bodies, under the influence of the loose mode of chemical combination spoken of as residual affinity, is carbon; so that a block of charcoal can absorb hundreds of times its own bulk of certain gases. Indeed, Sir James Dewar has recently employed this absorbing power of very cold carbon to produce a perfect kind of vacuum, which may, perhaps, be the nearest approach to absolute vacuum that has yet been attained: probably higher than can be attained by any kind of mechanical or mercury pump. _Unexpected Influence of Size._ Suppose now a substance contains a great number of carbon molecules and a great number of water molecules, each of which has this residual affinity or power of clinging together well developed, what may be expected to be the result? Surely, the formation of a molecule consisting of thousands or hundreds of thousands of atoms, constituting substances more complex even than those already known to or analysable by organic chemistry; and if these complex molecules likewise possess the adhesive faculty, a grouping of millions or even billions of atoms may ultimately be formed. (A billion, that is a million millions, of atoms is truly an immense number, but the resulting aggregate is still excessively minute. A portion of substance consisting of a billion atoms is only barely visible with the highest power of a microscope; and a speck or granule, in order to be visible to the naked eye, like a grain of lycopodium-dust, must be a million times bigger still.) Such a grouping is likely to have properties differing not only in degree but in kind from the properties of simple substances. For it must not be thought that aggregation only produces quantitative change and leaves quality unaltered. Fresh qualities altogether are liable to be introduced or to make their appearance at certain stages—certain critical stages—in the building up of a complex mass (_cf._ p. 71). The habitability of a house, for instance, depends on its possessing a cavity of a certain size; there is a critical size of brick-aggregate which enables it to serve as a dwelling. Nothing much smaller than this would do at all. The aggregate retains this property, thus conferred upon it by size, however big it may be made after that; until it becomes a palace or a cathedral, when it may perhaps reach an upper limit of size at which it would be crushed by its own weight, or at which the span of roof is too great to be supported. But the difference, as regards habitability, between a palace and a hovel is far less than that between a hovel and one of the air-holes in a brick or loaf, or any other cavity too small to act as a human habitation. The difference as regards habitability is then an infinite difference.