178 ' PHILOSOPHICAL TRANSACTIONS. [aNNO 1765. 



To which Dr. Huxham replied nearly as follows: " In many cases I have 

 known very good effects from a course of sea water, when drank in pretty large 

 quantities, and long continued; but it was when it purged gently, and now and 

 then puked somewhat. With the thin, tender, and hectical, it seldom agrees. 

 The gross,, heavy, and phlegmatic, commonly bear it with advantage. I have 

 known it bring on colic pains, diarrhoea, dysentery, and bloody stools, cough, 

 hectic heats, wasting of the flesh, and an hsemoptoe. It generally renders the 

 body liable to very great constipation, after it has been drank for a considerable 

 time." 



///. Experiments and Observations on the Agreement between the Specific Gra- 

 vities of the several Metals, and their Colours when united to Glass, as well as 

 those of their other Preparations. Ey Edward Delaval, F. R. S. p. 10. 



Sir Isaac Newton, in his optics, has showed, by a series of experiments, that 

 the several differences of colours, exhibited by thin transparent plates, are occa- 

 sioned by their several thicknesses ; and that therefore the transparent parts of 

 bodies do, according to their different sizes, reflect rays of one colour, and 

 transmit those of another ; and consequently that the size of the component 

 particles of natural bodies may be conjectured from their colours ; since the par- 

 ticles of those bodies most probably exhibit the same colours as a plate of equal 

 thickness, provided they have the same density. He concludes this whole doc- 

 trine in these words : " I have hitherto explained the powers of bodies to reflect 

 and refract, and showed that thin transparent plates, fibres, and particles, do, 

 according to their several thicknesses and densities, reflect several sorts of rays, 

 and thereby appear of several colours ; and by consequence, that nothing more 

 is requisite, for producing all the colours of natural bodies, than the several 

 sizes and densities of their transparent particles." 



Though he has accurately showed what colours arise from the several changes 

 of thickness, I do not find, says Mr. D., that any one has attempted to explain 

 in what manner the differences of density, in the component particles of bodies, 

 contribute to produce the several differences of colours : and therefore I thought 

 that if instances could be produced, of bodies whose several differences of colour 

 appear to be proportioned to their several degrees of density, it would tend to 

 illustrate this part of optics. To this purpose however are conducive all those 

 experiments and observations, from which Sir Isaac Newton has inferred that 

 bodies have their refractive and reflective powers nearly proportional to their 

 densities ; and that the least refrangible rays require the greatest power to reflect 

 them : which is deducible from hence, 1 . That the red rays are reflected at the 

 greatest obliquity of incidence, and the violet at the least. 2. That the violet is 

 reflected, in like circumstances, at the least thickness of any thin plate or bubble 



