1813.] Remarks on the Glohrics of Blood met Pus. 1 1 7 



particle detached within a vesicle, " like a pea in a bladder/' I 

 cannot doubt that Mr. Hewson was completely mistaken. I 

 have never observed a prominence in the outline of the particles 

 of the human blood : and on the other hand I am not perfectly 

 confident that the apparent depression, which is exhibited in 

 some lights, may not depend on some internal variation of the 

 refractive density of the particle. It has commonly been asserted, 

 that these coloured particles are readily soluble in water j but 

 this opinion appears to be completely erroneous, and to depend 

 partly on their passing readily through filtering paper, a circum- 

 stance indeed already observed by Berzelius, (Djurk. ii. p. 3,) and 

 partly on the extraction of a great part of their colouring matter, 

 together with which they lose much of their specific gravity, so 

 that instead of subsiding, they are generally suspended in the 

 fluid ; their presence may, however, still be detected by a careful 

 examination, and they seem in this state to have recovered in 

 some measure their original form, which they had lost when first 

 immersed in the water. When the water is sufficiently diluted, 

 about three-fourths as much rectified spirits may be added to it 

 without destroying the appearance ; but after a few months it 

 becomes indistinct, although neither in this case nor in that of 

 complete putrefaction do the globules appear to become consti- 

 tuent parts of a homogeneous fluid. The existence of solid 

 particles, in fluids which at first sight appear transparent, is the 

 most easily detected, by looking through them at a small lumi- 

 nous object, either directly or by reflection, as, for example, at 

 the image of a candle seen at the edge of the portion of the fluid, 

 held in a tea-spoon; in this case, wherever there are small par- 

 ticles in suspension, for instance, in milk diluted with water, they 

 will produce a minutely tremulous or sparkling appearance, 

 which is rendered still more distinct by the assistance of a lens, 

 and which depends on the diversified interception of the light, 

 while the particles are carried over each other by the internal 

 motion of the fluid. This test is applicable to all cases of minute 

 particles held in suspension ; where however the greater number 

 of the particles are nearly equal in dimensions, the luminous 

 object viewed through them exhibits a much more striking 

 appearance, for it is surrounded by rings of colours, somewhat 

 resembling those of the rainbow, but differently arranged, and 

 often beaulifully brilliant. The blood, a little diluted, always 

 exhibits them in great perfection, and they afford a very accu- 

 rate criterion for the distinction between pus and mucus: mucus, 

 containing no globules, affords no colours, while those which are 

 exhibited by pus exactly resemble the appearance produced by 

 the blood, the rings being usually of the same dimensions : 

 whence it follows that the globules are also of the same si/e, for 

 the dimensions of the rings vary with those of the particles 



