161 
frangible rays are at once stopped, and the red-ma- 
king rays, which are the least refrangible, are those 
only which pass; while, on the contrary, alkalis, 
which are supposed to precipitate and incrassate, and 
consequently to increase the resistance to the trans- 
mission of light, stop suddenly the red rays, and per- 
mit the green, and even the violet, the most refran- 
gible, and therefore the most easily intercepted of 
all the rays, to pass ; effects, in each instance, the re- 
verse of those which these agents ought to produce. 
The small portion, also, of acid or alkaline matter, 
that is required to change the colour of an infusion, 
can hardly be supposed to vary the density of the 
mixture so much as, by that mean alone, to produce 
such great alterations in its appearance : and this is 
still more difficult to conceive, when the addition is 
made, as it often is, by a body in a gaseous form. 
Lastly, the constitution of the fluid has itself under- 
gone a change, not merely from a change of density, 
but of composition also, which obliges us to take in- 
to account the chemical, as well as the mechanical 
effect. 
424. An observation, also, of Mr Delaval, points 
out a great dissimilarity in the action of different li- 
quors upon the rays of light. In Newton's experi- 
ment (419.), says he, it is probable that the liquor 
employed was an aqueous or spirituous infusion of 
the woods used in dyeing red, which transmits yellow, 
orange, or red colours, according to its thickness ; 
but the red solutions of flowers, and many others, 
do not transmit yellow or orange colours, even when 
spread thin ; for when thus disposed in thicker or 
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