INFLUENCE OF MECHANICAL SHOCK. 81 



One of the species was named by ENGELMANN (I.) Bacterium 

 phofometricum, on account of its variable susceptibility to the 

 colours of the spectrum and degrees of brightness. These 

 organisms, which will be fully noticed in a subsequent chapter, 

 also display the phenomenon known as movement of alarm. If a 

 microscopic preparation containing one of them in large numbers 

 be illuminated in such a manner that the light rays can fall 

 only on one sharply defined portion, then all the roving bacteria 

 collect within this space and bustle about briskly therein. If 

 now one of them in its onward career passes beyond the circle 

 of illumination into the dark portion, it stops instantly, and then 

 returns by the same road into the illuminated field. This is the 

 phenomenon of the movement of alarm. Consequently each 

 sharply defined illuminated portion of the field acts as a trap 

 for the bacteria, from which they cannot escape until the illumi- 

 nation has been altered. If a definite form be given to this trap, 

 such, for instance, as the shape of a W, and the closely con- 

 gregated cells be fixed and stained in this position, then a so-called 

 Bacterial photogram i.e. a coloured picture of the trap, composed 

 of the organisms themselves is obtained. 



63. Influence of Mechanical Shock. 



The first to inquire whether the vitality of lower organisms 

 can be influenced by agitation was A. HORVATH (I.) in 1878. 

 He made his observations with bacteria because he assumed that, 

 on account of their small size, the possibility of mechanical injury 

 (rupture) due to agitation would, in the case of these organisms, 

 be reduced to a minimum. On gently agitating bacterial cultures 

 (in Cohn's nutrient solution) he was unable to detect the mani- 

 festation of any retarding influence on the growth of the organism. 

 The results were, however, different when the sample was made 

 to undergo, by means of a shaking machine, about a hundred move- 

 ments in a direct line and of an amplitude of about 10 inches 

 (25 c.m.) per minute. This treatment for a period of twenty-four 

 .consecutive hours diminished the reproduction of the bacteria in 

 question ; and when continued for forty-eight hours, the agitation 

 proved fatal. On the basis of his researches Horvath formulated 

 the opinion that " for the development of the living organism, or 

 the physiological reproduction of the elements constituting the 

 organism, a certain degree of repose is necessary," meaning thereby 

 that rest mainly favours, whereas movement injures, reproduction. 

 This generalisation was opposed by NAGELI (II.) and E. Ch. 

 Hansen, the former of whom drew attention more particularly to 

 ihe reproduction of algae living beneath large waterfalls and 

 exposed to much more violent agitation than was effected by 

 Horvath's shaking apparatus. 



In 1879 E. CH. HANSEN (I.) instituted experiments in order 



VOL. I. F 



