Among the smallest of known organisms we have, for instance, _Micromonas mesnili_, Bonel, a flagellate infusorian, which measures about ·34 _µ_, or ·00034 mm., by ·00025 mm.; smaller even than this we have a pathogenic micrococcus of the rabbit, _M. progrediens_, Schröter, the diameter of which is said to be only ·00015 mm. or ·15 _µ_, or 1·5 × 10^{−5} cm.,—about equal to the thickness of {39} the thinnest gold-leaf; and as small if not smaller still are a few bacteria and their spores. But here we have reached, or all but reached the utmost limits of ordinary microscopic vision; and there remain still smaller organisms, the so-called “filter-passers,” which the ultra-microscope reveals, but which are mainly brought within our ken only by the maladies, such as hydrophobia, foot-and-mouth disease, or the “mosaic” disease of the tobacco-plant, to which these invisible micro-organisms give rise[65]. Accordingly, since it is only by the diseases which they occasion that these tiny bodies are made known to us, we might be tempted to suppose that innumerable other invisible organisms, smaller and yet smaller, exist unseen and unrecognised by man. [Illustration: Fig. 2. Relative magnitudes of: A, human blood-corpuscle (7·5 µ in diameter); B, _Bacillus anthracis_ (4 – 15 µ × 1 µ); C, various Micrococci (diam. 0·5 – 1 µ, rarely 2 µ); D, _Micromonas progrediens_, Schröter (diam. 0·15 µ).] To illustrate some of these small magnitudes I have adapted the preceding diagram from one given by Zsigmondy[66]. Upon the {40} same scale the minute ultramicroscopic particles of colloid gold would be represented by the finest dots which we could make visible to the naked eye upon the paper. A bacillus of ordinary, typical size is, say, 1 µ in length. The length (or height) of a man is about a million and three-quarter times as great, i.e. 1·75 metres, or 1·75 × 10^6 µ; and the mass of the man is in the neighbourhood of five million, million, million (5 × 10^{18}) times greater than that of the bacillus. If we ask whether there may not exist organisms as much less than the bacillus as the bacillus is less than the dimensions of a man, it is very easy to see that this is quite impossible, for we are rapidly approaching a point where the question of molecular dimensions, and of the ultimate divisibility of matter, begins to call for our attention, and to obtrude itself as a crucial factor in the case. Clerk Maxwell dealt with this matter in his article “Atom[67],” and, in somewhat greater detail, Errera discusses the question on the following lines[68]. The weight of a hydrogen molecule is, according to the physical chemists, somewhere about 8·6 × 2 × 10^{−22} milligrammes; and that of any other element, whose molecular weight is _M_, is given by the equation (_M_) = 8·6 × _M_ × 10^{−22}. Accordingly, the weight of the atom of sulphur may be taken as 8·6 × 32 × 10^{−22} mgm. = 275 × 10^{−22} mgm.