96 SOLID CULTIVATION MEDIA. 



solid, and any organism developing remains with its progeny and its 

 products in one position, and " one sees at a glance the number exist- 

 ing in the water." 



In very impure water the organisms which are found in it in great 

 abundance appear to require the presence of oxygen for their growth, 

 and therefore liquefaction of the gelatine takes place only at the 

 surface, and the deeper parts may appear to contain fewer active 

 organisms than some water that is comparatively pure, in which " the 

 whole water becomes filled with perfectly formed and transparent 

 spheres, forming very beautiful objects. At the bottom of the spheres 

 is a little white line ; sometimes this becomes rather marked, and if 

 it is too heavy, it sinks below the circumference, and gives an appear- 

 ance of a balloon with a parachute." 



" If a pipette is put into these spheres, and suction applied, it will 

 be found that they are full of liquid, whilst the gelatine around is 

 solid. The deposit below contains a great mass of active and 

 inactive bacteria." 



" There is another phenomenon appearing in some of them, viz. — 

 a number of very minute white dots, which increase to a size of a 

 pin-head," but which do not liquefy the gelatine. "These may be 

 scattered in countless numbers through the water, and seem to 

 indicate the number of points of vitality in the water." 



Dr. Smith considers that gelatine alone gives sufficiently good 

 results, but that phosphate of soda or sugar, or both, may be added 

 to the gelatine, in which cases the changes go on more rapidly, even 

 too rapidly for accurate observation. This difficulty, however, is easily 

 overcome by taking a smaller quantity of water, so that the spherules 

 may be less numerous. 



The great advantage that this process has over the flat glass plate 

 method is, that observations may be carried on in the same specimen 

 for a very considerable period, the large mass of gelatine affording 

 ample space for extension in all directions, in addition to which there 

 is not any great tendency to dry, as there is in the case of the thin 

 film of gelatine on the glass plate. On the other hand, it is impos- 

 sible to get secondary inoculations on to other media by Dr. Smith's 

 method, though he suggests that part of the surface growth may be 

 diluted with distilled water and added to a fresh quantity of gelatine. 



