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SCIENTIFIC NEWS. 



[Nov. 30, if 



he must now call attention to the fact that there were 

 many curious physical differences between quartz, 

 or co'stalline silica, and colloidal silica. The first was 

 the difference of density, or weight, to which he must 

 call their attention ; and in comparing those substances 

 they must compare either equal bulks of substance or else 

 refer both to one common standard, and the standard 

 which was chosen for comparing weights of solid sub- 

 stances was water. Now if he were to take a small cubical 

 vessel of such a size that when it was filled it would con- 

 tain just one pound of water, and then take a piece of 

 quartz and cut it out in a block of exactly the same 

 size as the interior of the vessel, so that it would fit 

 into it and fill it exactly, if we weighed that quartz it 

 would be found that it weighed considerably over a 

 pound — that it would weigh exactly 2| lbs., or putting 

 it in decimal instead of vulgar fractions, 2 - 66. Now, if we 

 took the other variety, the colloidal silica, and treated 

 that in the same way, it would weigh 2j lbs., or in deci- 

 mals 2'2. Now they saw that these two substances differed 

 in a remarkable manner in their density orspecific gravity. 

 As a matter of fact, it was a very troublesome opera- 

 tion, as they might imagine, to cut blocks of colloidal 

 silica, or quartz, into definite forms to weigh them, 

 and they could arrive at the specific gravity by a very 

 much easier method. If we take a block of crystal- 

 line silica and weigh it carefully in a delicate balance 

 in the air, and get its weight, and then take the 

 same mass and sustain it in the vessel of water 

 while it is being weighed, it will be found that 

 the water will buoy up the mass, and that it 

 will weigh less than when it was weighed in the 

 air. It had been found that the diminution of weight 

 when it was weighed in water was equal exactly to the 

 weight of the bulk of the water which the block dis- 

 placed. That very remarkable discovery is ascribed to 

 Archimedes. We can determine the specific gravity 

 of a body at once by making use of this principle. 

 If the crystalline silica was weighed in air and 

 then in water, they would have the weight of the 

 displaced volume of water, and if we divide it into 

 the weight of the substance we shall get the specific 

 gravity. If we treat a piece of flint in that way it 

 will be found that the specific gravity is always 

 something less than that of quartz, or more than 

 that of colloidal silica. Flint has a density between that of 

 quartz and colloidal silica. Now, if the matter was made 

 up of atoms or molecules — molecules being built 

 up of atoms — and if the atoms and the molecules of 

 the same substance have the same weight, then, 

 knowing that colloidal silica and quartz, or crystalline 

 silica, were built up of the same elements in the same 

 proportions, they would arrive at the conclusion that if 

 the one substance was lighter than the other it must be 

 because there are more particles in a given space in one 

 case than in the other ; they would arrive at the 

 conclusion that in colloidal silica the molecules did 

 not lie so close to one another as they did in crystalline 

 silica. And there were many facts which tended to bear 

 out that conclusion. The first which he would call their at- 

 tention to was this : If he took a piece of quartz and put 

 it into water or any coloured liquid, such as ink for ex- 

 ample, it would be found there was no tendency for the 

 liquid to be absorbed by the substance crystalline silica. 

 The ink could be washed off the surface, and it would 

 seem to have undergone no kind of change. There was 

 no tendency in crystalline silica to absorb the coloured 



liquid ; there was no evidence that it was in any 

 degree porous. But if they took a piece of colloidal silica 

 and put it into liquid they would soon find that the 

 colloidal silica tended to absorb the liquid without under- 

 going any change in its own bulk, that it exhibited 

 evidence of porosity; and if they used a coloured liquid 

 for experiment they would see that fact proved by the 

 circumstance that the coloured liquid would permeate 

 the whole mass of the colloidal silica. When he said 

 that crystalline silica did not absorb a coloured liquid 

 he did not refer to the existence of exceptional cracks 

 such as were to be found in specimens of crackle quartz. 

 If the crackle quartz were put into a coloured liquid 

 the liquid would find its way into the cracks, and then 

 it might give a portion of it a pretty appearance, but the 

 liquid only penetrated along the cracks, and not into the 

 substance of the mass. But colloidal silica appeared to 

 be an absolutely porous substance, though some forms 

 of colloidal silica were much more porous than other 

 forms. One of the most porous forms of colloidal silica 

 was a very singular form of substance known by the 

 name of tabasheer. Tabasheer is the Arabic name 

 for a very curious substance, which had a singular origin. 

 They had heard that silica passed into solution under 

 various conditions, and in quarters of the globe there 

 was a certain quantity of silica in solution. Silica was 

 taken up by the roots of plants, especially by certain 

 varieties of plants, such as the grasses, and these 

 grasses were capable of absorbing this colloidal silica, 

 and building it up into their structures. Many of 

 the grasses contained a considerable quantity of silica, 

 in their stems especially, and sometimes particles 

 ot it exuded from the surface of the stem. There 

 was a gigantic grass — the bamboo — that absorbs silica, 

 like all other grasses, and builds silica into its 

 structure. When the grass on the bamboo was 

 burnt they sometimes got a considerable quantity 

 of silica. Now he had to refer to a curious circum- 

 stance that sometimes occurred in the case of bamboos. 

 Bamboo, like other grasses, consist of a number of 

 joints which were cut off from one another, there being 

 a central hollow cavity between the separate joints. 

 Those rings were solid, and the spaces between were 

 hollow. Now it appeared that when bamboos were in 

 an unhealthy condition or had suffered from some kind 

 of injury, the silica which was taken up in them might 

 accumulate in those hollow portions of the stem, and 

 thereby they got deposits of hard silica in the bamboo. 

 It was only now and then that it is found. From very 

 early times the Eastern peoples had been in the habit of 

 looking for these deposits of silica in the joints of the 

 bamboo, and they gave to it the name tabasheer. It 

 was very highly prized by Arab physicians ; and was 

 taken internally. One did not know what good or harm 

 it was likely to do ; but he did not know that that might 

 not be true of certain Western medicines. However, 

 that substance was highly prized, and was sold in all 

 Eastern markets. Now, this tabasheer was a curious 

 form of colloidal silica. It was so porous that the pores 

 were two and a half times the bulk of the solid silica 

 which enclosed them. They very seldom found any 

 form of colloidal silica so excessively porous as 

 that, though all forms of colloidal silica were more 

 or less porous. Now let them see how they 

 could prove the porous character of the substance 

 tabasheer. Sir David Brewster, who was the first to 

 call attention to the curious properties of tabasheer, indi- 



