VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 233 



not a sufficient cause for the production of the colour which they occasion ; for the 

 colour of blood, after those substances have acted on it, is a scarlet, which, agree- 

 ably to the observation of Dr. G. Fordyce, (Elements of the Practice of Physic, 

 p. 13) differs not only in brightness, but also in kind, from the ordinary colour of 

 that fluid, which is a Modena red. 



My answer is, that there are examples, besides that to which the objection is 

 made, of dark blood appearing florid, merely from its colouring matter being seen 

 by means of an increased quantity of light. One is afforded by rubbing a piece of 

 the darkest crassamentum with a proper quantity of serum; for a mixture is thus 

 formed, in a few seconds, possessing a colour similar to that which is given to cras- 

 samentum by common air. But here we certainly do nothing more than interpose, 

 among the red globules, a number of the less dense particles of serum; which, in 

 their present situation, act as opake matter, and consequently increase the internal 

 reflections. A 2d example occurs, when we view, by transmitted light, the fine 

 edges and angles of a piece of crassamentum in water; for, in this situation, their 

 colour appears to be a bright scarlet, though all the other parts of the same mass 

 are black. These facts seem sufficient to prove, that the immediate cause I have 

 assigned for the production of the florid appearance in blood, which has been ex- 

 posed to the action of common air and neutral salts, is adequate to the effect; but 

 I shall advance a step farther, and show how the Modena red is converted into a 

 scarlet. Blood, as I have found by experiment, is one of those fluids which Sir 

 Isaac Newton has observed appear yellow, if viewed in very thin masses; book ], 

 part 2, prop. 10. When therefore a number of opake particles are formed in it, 

 by the action of common air and the neutral salts, many of them must be situated 

 immediately beneath the surface. The light reflected by these will consequently be 

 yellow ; and the whole effect of the newly-formed opake particles, on the appearance 

 of the mass, will be the same, as if yellow had been added to its former colour, a 

 Modena red. But Modena red and yellow are the colours which compose scarlet; 

 Fordyce's Elements of the Practice of Physic, p. 14. 



I shall now relate the cautions to be observed in making the experiments, which 

 are described in the beginning of this paper. The first is, that the blood should be 

 newly drawn, and the weather cool. For as the solution of the red matter is not 

 to be filtred, but must become transparent by the gradual subsiding of whatever 

 may render it turbid, if the blood be old, or the weather warm, it will often assume, 

 before it be clear, a dark and purplish hue. When exposed in this state to the at- 

 mosphere in a broad and shallow vessel, its colour changes to a bright red, which 

 however is not brighter than the proper colour of the solution. The dark purplish 

 hue seems owing to some modification of sulphur; for the solution possessing it 

 smells like hepatic air, particularly when agitated, and tarnishes silver held over it. 

 Neutral salts produce no change on this colour. 



The 2d caution is, that the neutral salts be not added to the red solution, except 

 when perfectly transparent; for if it be not so, the salts will render it more turbid, 



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