426* Dr. Wells’s Observations and Experiments 
formed opake particles, upon 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. * 
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 sub- 
siding 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 
atmosphere in a broad and shallow vessel, its colour changes 
to a bright red, which, however, is not brighter than the pro- 
per 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 which is held over it. Neutral salts produce no change 
upon this colour. 
The second 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, and the mixture 
will appear brighter, if seen by reflected light. 
The last I shall note is, that the red solution ought to be 
poured gently from the vessel in which it has been made. If 
it be not, as it is a mucilaginous liquor, it is apt to entangle 
small particles of air, which by acting as opake matter, will for 
some time alter the appearance of the solution. 
* Fordyce’s Elements of the Practice of Physic, p. 14. 
