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on the Latent State of Light. These negative actions are never 

 produced by any of the rays of solar light. 



" It appears to me that these new facts, of which I have communi- 

 cated to you the most remarkable, lead to important results. It 

 naturally follows from them, that the oxygen of the atmosphere will 

 retard the action of light, and tend to preserve the surfaces of bodies 

 in their primordial condition, I have, without success, endeavoured 

 to make a direct experiment on this subject. I placed a camera- 

 obscura under the receiver of an air-pump, and exhausted the air ; 

 the time, however, for producing any particular effect of light did 

 not change ; but the air-pump was so imperfect, that it did not pro- 

 duce a rarefaction beyond five or six inches of the barometer. In 

 the mean time, however, I may adduce some of my experiments 

 comnmnicated seven months ago to the Academy of Sciences at Ber- 

 lin, which prove, at least indirectly, what the air-pump has not yet 

 verified. If we wet an iodised plate of silver with any fat oil, we 

 shall find that the time in which light produces any particular effect 

 is greatly diminished. In applying, for example, oil of olives, the 

 plate will be four or five times more sensitive ; so that, instead of four 

 or five minutes, one minute only will be necessary to produce the 

 same effect in the camera. The mercurial vapour readily penetrates 

 the thin film of oil, and produces very perfect images. This singu- 

 lar action of oils arises probably from their obstructing, or, at least, 

 enfeebling, the negative action of oxygen gas. The mercurial vapours 

 produce even a better developed image, that is to say, one which cor- 

 responds to a more prolonged action of light, if wo apply the oil only 

 at the instant the plate is taken out of the camera. 



" I believe that these negative actions, which brino- back the surfaces 

 of bodies to their natural state, will throw some light upon the pro- 

 cess of vision, because the same effects seem to take place upon the 

 retina. But I greatly regret that I am not able to enter into any 

 details on this mteresting subject, as I am wholly occupied with other 

 researches, particularly on a very remarkable and different kind of 

 action exerted by hydrogen gas." 



3. On the Specific Gravity of certain Substances commonly 

 considered lighter than Water. By Dr Davy. 



The author enters on the subject by adverting to the apparent 

 lightness imparted to the common woods, and to certain vesicular 

 minerals, by the entanglement of air in their substance, as is com- 

 monly understood, and as is proved by the action of the air-pump. 



