76 Nitrous Oxide.-^Dog^s Mercury; 



In our ample experience^ we have not found the experiment to fail above once In twenty 

 times. With a given degree of accuracy and perfeverance, it would, I apprehend, never 

 fail. I do not mean that the refult would be always agreeable. 



The gas being procured genuine, the lungs fliould be emptied by a ftrong expiration, 

 the nofe held tight, and the lips exadlly Clofed on the mouth-piece. From 5 to 8 quarts 

 breathed backwards and forwards into the refervoir (a proceeding of which Mr. Davy's 

 refearches will fiiew the propriety) have proved an average dofe. The feelings will deter- 

 mine the time. To the hyfterical and the exquifitely fenfible, the gas fhould not be ad- 

 miniftered. 



It is very well to know from aftual trial, what the London experiments prove, that in- 

 terrupted refpirations, during which the gas is probably much diluted with refiduary air in 

 the lungs, can feldom have efFed. This, however, an obferver of any fagacity could 

 have predifted. 



A laborious analyfis, and the refult of a vaft variety of careful obfervations, will ere long 

 appear, to corre£t the mifconceptions which may have arifen from loofe cafual trials. 

 Meanwhile, I beg you to Infert this explanation in your valuable Journal. I think It of no 

 fmall importance to mankind, that there fhould exift nothing to prevent this great agent 

 from being employed, whenever diredt experience or circumfpedt analogy warrants its ufe. 



I am, Sir, 



Refpedfully your's, 



THOMAS BEDDOES. 



VI. 



6n the colouring Matter of Dog's Mercury (Mercurialis perennis Lin.) By a Correfpondent. 



To Mr. NICHOLSON. 

 SIR, 



Ri 



RESEARCHES concerning the colouring principles of vegetable fubftances, have been 

 very general and extenfive ; but I have never read any thing concerning the colour 

 which refides in the juice of dog's mercury, and therefore conclude, that the properties 

 of this vegetable are not very generally known. 



The herb grows plentifully in woods and hedges. Its medical virtues are at prefent 

 negledted 5 and It Is never removed from its folltary abode for any ufeful purpofe, but the 

 inftruftion of the botanlft. Accident may, perhaps, have difclofed this fa£l: to him, that 

 its juice affords a blue ftain or colour : any one, however, may afcertain its truth by the 

 following eafy experiment : 



Tal.:e 

 4 



