gas from another, and almost the only quality noted is a want of
permanence, _e.g._ when the CO2 produced was dissolved by the water over
which he collected it. Sir E. Thorpe {134a} points out that Hales must
have prepared hydrogen, carbonic acid, carbonic oxide, sulphur dioxide,
and marsh gas. It may, I think, be said that Hales deserved the title
usually given to Priestley, viz. "the father of pneumatic {134b}
chemistry."

Perhaps the most interesting experiment made by Hales is the heating of
minium (red-lead) with the production of oxygen. It proves that he knew,
as Boyle, Hooke and Mayow did before him, that a body gains weight in
oxidation. Thus Hales remarks: "That the sulphurous and aereal particles
of the fire are lodged in many of those bodies which it acts upon, and
thereby considerably augments their weight, is very evident in Minium or
Red Lead, which is observed to increase in weight in undergoing the
action of the fire. The acquired redness of the Minium indicating the
addition of plenty of sulphur in the operation." He also speaks of the
gas distilled from minium, and remarks: "It was doubtless this quantity
of air in the Minium which burst the hermetically sealed glasses of the
excellent _Mr. Boyle_, when he heated the Minium contained in them by a
burning glass" (p. 287).

This was the method also used by Priestley in his celebrated experiment
of heating red-lead in hydrogen, whereby the metallic lead reappears and
the hydrogen disappears by combining with the oxygen set free. This was
expressed in the language of the day as the reconstruction of metallic
lead by the addition of phlogiston (the hydrogen) to the calx of lead
(minium). Thorpe points out the magnitude of the discovery that
Priestley missed, and it may be said that Hales too was on the track, and
had he known as much as Priestley it would not have been phlogiston that
kept him from becoming a Cavendish or Lavoisier. What chiefly concerns
us, however, is the bearing of Hales' chemical work on his theories of
nutrition. He concludes that "air makes a very considerable part of the
substance of Vegetables," and goes on to say (p. 211) that "many of these
particles of air" are "in a fixt state, strongly adhering to and wrought
into the substance of" plants. {135a} He has some idea of the
instability of complex substances, and of the importance of the fact, for
he says {135b} that "if all the parts of matter were only endued with a