306 
PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
air which had produced the observed absorption, and found to be as dry as polished 
plate glass. For a week at a time I have charged my experimental tube alternately 
with dried and undried air, removing every evening the plates of salt while the humid 
air filled the tube. Their dryness and polish were found unimpaired/'' I have 
frequently flooded the experimental tube with light, and watched narrowly whether 
any dimness showed itself on the salt, or on the interior surface, when the humid air 
entered. There was nothing of the kind. I finally abandoned the plates of salt 
altogether, and obtained in a tube open at both ends substantially the same effects as 
those obtained when the tube was closed with plates of rocksalt. 
In 1862 Magnus came to London. He had been previously working at the points 
of difference between us, and had strengthened his first conviction. The action of the 
air he had found to be considerable, and the action of aqueous vapour practically nil. 
I also had been working, but with an entirely different result. It was hoped by both 
of us that our differences would be settled during this visit. With my closed experi¬ 
mental tube I showed him the neutrality of dry air and the activity of humid air; and 
while the latter was in the tube I detached the rocksalt plates and placed them in his 
hand. He closely inspected them, passed his dry handkerchief over them, and frankly 
and emphatically pronounced them perfectly dry. I then executed in his presence the 
experiments with the open tube, and reproduced the results which I had previously 
published. I subjected the method of compensation to a severe test and showed him 
how exact it could be made. He frankly confessed his inability to find any flaw in 
my experiments, and save in one particular made no attempt to reconcile our differ¬ 
ences. He accounted for the neutrality of dry air observed by me by pointing to my 
thermopile, between which and the experimental tube a space of air intervened. He 
argued, and justly argued, that though the calorific rays were permitted to enter the 
tube from a vacuum, if the air intervening between tube and pile could produce the 
effect which he ascribed to it, the heat would be robbed of its absorbable rays before the 
dry air entered the tube, the subsequent neutrality of dry air being a matter of course. 
The logic was good, but its basis I knew to be more than doubtful; and I therefore asked 
him whether a layer -^th of an inch thick between pile and tube would produce any 
sensib]e effect. His reply was an emphatic negative. In subsequent experiments, 
therefore, the conical reflector was removed from my pile, and placed within the experi¬ 
mental tube, its narrow end being caused to abut against the plate of rocksalt. The 
face of the pile was then brought within less than ^ 0 -th of an inch of the rocksalt 
plate; and in this way my former measurements, which had declared the pure air of 
our atmosphere to be a practical vacuum to radiant heat, were verified to the letter. 
The well-earned fame of Magnus as an experimenter, and his personal friendliness 
to myself, rendered it specially incumbent on me to deal respectfully with every one of 
his suggestions. He once intimated to me that the absorption which I had supposed 
* This mastery over the apparatus was not attained without training. Any lapse of care soon declared 
itself by the condition of the plates of salt. 
