18 
POPULAR SCIENCE REVIEW. 
to Melobesia calcarea. The same observer has likewise con- 
tributed to the knowledge of sponges by an interesting experi- 
ment. He fed a marine calcareous sponge with indigo, examined 
it at the moment, and then preserved it in spirit, where he says 
“ it now shows all the cells (monociliated) with the cilium 
attached, and the indigo still in the cells.” 
From Professor Norris, of Queen’s College, Birmingham, we 
learn that not only do blood corpuscles pass through the walls 
of blood-vessels, as previous observers had seen, but that some 
explanation may be offered of the puzzling fact of their doing so 
without the previous existence of apertures, or any trace thereof 
after they have passed through. He arrives at the following 
generalization : That a rigid or plastic body can pass through 
a colloid film, if there is, first, an intimate power of cohesion ; 
secondly, a certain amount of pressure from within ; thirdly, 
power of the substance of the film to cohere to the surface of 
the body, or to some intermediate matter which already coheres 
to the surface during its passage ; and fourthly, cohesive plas- 
ticity of the particles of the material of which the film itself is 
composed, so that the breach in it may again become united 
as it descends upon the opposite surface of the body which is 
being extended. This is a view of the matter that requires to 
be supported by experiments with colloid films. 
The mode in which silica is deposited in various plants, and 
especially in diatoms and others, which exhibit that mineral in 
regular patterns, has engaged the writer’s attention, and he 
regards it as probable that the deposit usually takes place, not 
from the decomposition of an alkaline silicate, as usually sup- 
posed, but from a solution of pure silica in the colloid state, 
dissolved by rain-water. Such a deposit taking place slowly, 
through a plant membrane or on its surface, might be expected 
to assume the form of minute spheres, separated or coalescing 
according to the rapidity of the process, the quantity of silica 
in solution, and the exact nature of the surroundings. Quick 
deposits from rich solutions would probably lead to coalescence 
in amorphous forms, as seen to a remarkable extent in the bark 
of the curious pottery-tree, where it is found in lumps. If 
these views are correct, it is probable that all diatoms have 
their siliceous skeletons composed of spherules ; and a careful 
examination not only of the so-called “ costae,” but also of some 
of the apparently plane and homogeneous surfaces of pinnulariae, 
lead to the conclusion that such is their structure. The 
so-called “costae” are not solid ribs at all, but curious and 
complicated beaded structures. These views were brought 
before the Boyal Microscopical Society in a paper published in 
the August number of the Journal. 
The writer also called attention to the modifications of crys- 
