196 R. j. Harvey Gibsoii. 
in the water in some vessels which he had used for them, and that 
it gave off a very ‘pure air’; a number of further observations 
taught him that this air was given off only under the influence of 
sunlight; Priestley himself did not suspect that the deposit in 
question, afterwards known as * Priestley’s matter ’ and found to 
consist of Algae, was a vegetable substance.” Hansen (“ Geschichte 
der Assimilation,” Arb. bot. Inst. Wurzburg, 1884) also, no doubt 
following Sachs’s lead, writes:—“ Priestley behauptete, die Substanz 
des Bodensatzes sei weder vegetablisch, noch animalisch, sondern 
eine Substanz eigener Art welche er grime Materie nannte.” 
These views are scarcely substantiated by a perusal of Priestley’s 
“ Experiments and Observations on Different Kinds of Air,” 
published in 1790. Book IX of this famous classic is devoted 
entirely to “ observations and experiments relating to vegetation 
and respiration ” and in it he describes how Mr. Walker, of “ English 
Dictionary ” fame, told him that while waiting for a boat to convey 
him to the Continent, he noted a horse-trough at the principal Inn 
at Harwich, which the Inn-keeper refused to have cleaned out 
because he had found that its contents remained sweet when 
the sides and bottom were “covered by a green substance which is 
known to be of vegetable nature.” Section VI of the same Book 
deals with the “ spontaneous emission of dephlogisticated air from 
water containing a vegetative green matter ” and in the body of the 
section he states that he “ never found it [to give off the gas] 
except in circumstances in which the water had been exposed to 
light.” In Section VII he says he at first supposed the green 
substance to be a plant but was “ unable to discover the form of 
one. Several of my friends, however, better skilled in Botany than 
myself, never entertained any doubt of its being a plant; and I had 
afterwards the fullest conviction that it must be one. Mr. Bewley 
has lately observed the regular form of it by a microscope. My 
own eyes having always been weak I have, as much as possible, 
avoided the use of the microscope.” In the following section, 
Priestley adds, “ It will come most properly under the denomination 
of the Conferva ; but this not being within my province I shall not 
presume to give it any particular appellation.” Priestley’s 
modesty is thus translated by Sachs into ignorance : for he not 
only “ suspected ” but afterwards “ had the fullest conviction ” that 
the green matter which gave off “ pure air” in his tanks was a 
plant but was content, though not a professed botanist, to regard it 
as a green Alga! After reading these sections in the “ Experiments 
