the late Mr. Henry. 7 



the publicateons of his friends Dr. Priestley and Dr. Percival. 

 Of these, the most important were some Experiments on the 

 Influence of Fixed air on Vegetation, by which he endeavoured 

 to shew that though fixed air ie injurious, when unmixed, to the 

 vegetation of plants, yet that when mingled in small proportion 

 with common air, it is favourable to their growth and vigour. 

 The facts established by this inquiry, were communicated to Dr. 

 Priestley ; and it is creditable to the candour of that distinguished 

 philosopher, that he was anxious to make them public, not only 

 for their general merit, but because in one or two points the 

 results disagreed with his own. " I am much pleased," Dr. 

 Priestly replies, " with the experiments mentioned in your letter, 

 and if you have no objection, shall be glad to insert the greatest 

 part of it in my Appendix, which I am just sending to the 

 printer's. I the rather wish it, as a few of the experiments ter- 

 minate differently from those that I shall publish, and I wish to 

 produce all the evidence I can come at on both sides. The 

 other experiments are very curious and will give much satisfac- 

 tion*." The investigation was afterwards resumed by Mr. 

 Henry, and made the subject of a paper, which is printed in the 

 second volume of the Memoirs of this Society. 



The occasion of Mr. Henry's next appearance, as the author 

 of a separate work, arose out of an accidental circumstance. 

 He had found that the water of a large still tub was preserved 

 sweet for several months by impregnating it with lime, though, 

 without this precaution, it soon became extremely putrid. This 

 fact suggested to him an eligible method of preserving water at 

 seat; but as lime water is unfit for almost every culinary pur- 

 pose, some simple and practicable method was required of 

 separating that earth from the water, before being applied to 

 use. This he ascertained might be accomplished at little ex- 

 pence by carbonic acid, the gas from a pound of chalk and 12 



* Letter from Dr. Priestley to Mr. Henry, dated Jan. 5, 1777. 



t Dr. Alston of Edinburgh appears, however, to have been the first 

 wlio pro])oscd impregnation with lime, as a mean of prevcniing tlic putre- 

 faction of water ; and to precipitate the lime, he suggested the use of 

 carbonate of magnesia. 



