1895.] for the estimation of the Specific Oravity of Tissues. 281 



and almost completely comes to rest. This layer corresponds to 

 the specific gravity of the tissue. 



The advantages of the method so far as concerns the tissue 

 have already been given, it is necessary to notice how the actual 

 fluids behave and especially how far they diffuse into one another. 

 If such a column of fluid be made and left undisturbed the bands 

 will be absolutely distinct after 24 hours though the edges 

 of course are not so clearly defined, while after 48 hours it ob- 

 viously consists of deeper and paler bands of blue. Since even 

 a long experiment of the kind in which the method would be 

 employed rarely lasts above 8 hours at the most, diffusion of the 

 layers of fluid into one another may be neglected \ 



It is plain that when several pieces of tissue have been placed 

 in the same column of fluid there is a tendency for portions of the 

 upper layers to be dragged downwards by the falling mass of 

 tissue and therefore for the layers to become mixed, but it is 

 remarkable how little of the superincumbent layers is thus carried 

 down. This can be seen by watching a piece of tissue in its 

 course through the yellow bands. A small amount of the super- 

 jacent blue band is carried down, but this rapidly returns to 

 its own proper region by reason of its inferior specific gravity 

 so that it is quite possible to take ten or twelve estimations in 

 the same column of fluid. The principal disadvantage in the 

 method is that diffusion of water though slow nevertheless takes 

 place from the muscle into the gum solutions and therefore that 

 the piece of muscle does not absolutely come to rest until it 

 reaches the bottom of the test-tube. After a little practice how- 

 ever it is quite possible to distinguish a difference of one degree 

 in specific gravity. Since the differences actually noticed in 

 experiment vary from about two degrees to eight or ten degrees 

 when variations do occur, the disadvantage is not nearly so serious 

 as might appear, while the possible margin of error is not so great 

 with gum solutions, since a piece of muscle takes as many minutes 

 to sink through the same number of degrees after it has reached 

 that layer which coincides with its own specific gravity as in 

 the case of glycerine solutions it takes seconds. 



^ The possibility that diffusion of the colour might not be an indication of the 

 diffusion of the solutions was borne in mind, and therefore by small pipettes fluid 

 was taken from the various layers after the column had stood for some time and 

 the specific gravity of each estimated by Roy's method. It was found that this 

 possible source of error does not exist, and that after several hours the specific 

 gravity of any individual layer is the same as it was when it was first placed in the 

 column, provided the fluid taken up in the pipette did not come from either the 

 upper or lower margin of the layer. 



