[Illustration: NORMAL AND PLEON-FORMS OF THE BACILLUS OF TUBASH.] Here, then, we have the major parts in the constitution of a bacillus--its body, mycoprotein; its capsule, cellulose. But, further than this, there are a number of additional distinctive characteristics as regards the contents inside the capsule which call for mention. _Sulphur_ occurs in the Beggiatoa which thrive in sulphur springs. _Starch_ is commoner still. _Iron_ as oxide or other combination is found in several species. Many are highly coloured, though these are generally the "innocent" bacteria, in contradistinction to the disease-producing. A pigment has been found which is designated _bacterio-purpurin_. According to Zopf, the colouring agents of bacteria are the same as, or closely allied to, the colouring matters occurring widely in nature. Migula holds that most of the bacterial pigments are non-nitrogenous bodies. There are a very large number of chromogenic bacteria, some of which produce exceedingly brilliant colours. Among some of the commoner forms possessing this character are _Bacillus et micrococcus violaceus_ (violet); _B. et M. aurantiacus_ (orange); _B. et M. luteus_ (yellow); _M. roseus_ (pink); many of the _Sarcinæ_; _B. aureus_ (golden-yellow); _B. fluorescens liquefaciens et non-liquefaciens_ (green); _B. pyocyaneus_ (green); _B. prodigiosus_ (blood-red). _Motility._ When a drop of water containing bacteria is placed upon a slide, a clean cover glass superimposed, and the specimen examined under an oil immersion lens, various rapid movements will generally be observed. These are of four kinds: (1) A dancing stationary motion known as _Brownian movement_. This is molecular, and depends in some degree upon heat and the medium of the moving particles. It is non-progressive, and is well known in gamboge particles. (2) An _undulatory_ serpentine movement, with apparently little advance being made. (3) A _rotatory_ movement, which in some water bacilli is very marked, and consists of spinning round, with sometimes considerable velocity, and maintained for some seconds or even minutes. (4) A _progressive_ darting movement, by which the bacillus passes over some considerable distance. The conditions affecting the motion of bacteria are but partly understood. Heating the slide or medium accelerates all movement. A fresh supply of oxygen, or indeed the addition of some nutrient substance, like broth, will have the same effect. There are also the somewhat mysterious powers by which cells possess inherent attraction or repulsion for other cells, known as _positive_ and _negative chemiotaxis_. These powers have been observed in bacteria by Pfeiffer and Ali-Cohen. [Illustration: BACILLI, SHOWING FLAGELLA]