WITHDRAWN FROM THE ACTION OF GRAVITY. 211 



would be vertical, and enlarges in appearance tlie horizontal dimensions of the 

 object. * 



In order entirely to avoid this illusion, we must use a vessel of plane smooth 

 sides, formed of plates of glass set in a metal frame, (§ 8.) We then have, in a 

 complete manner, the curious spectacle of a considerable mass of liquid present- 

 ino- the. form of a perfect sphere, and imitating, in some measure, a planet sus- 

 pended in space. 



Instead, also, of the above vessel, a glass balloon may be used, which is more 

 imple and less expensive. In this case, indeed, the mass of oil only appears 

 in its real figure when it occupies the centre of the balloon ; but the apparent 

 distortion is small, as long as the sphere is not moved considerably from this 

 centre. A vessel of this kind is very convenient for most of the experiments 

 which I shall describe in this part of the memoir ; but it would not serve for 

 those which I shall have to make known subsequently". 



6. Now, having obtained, by means of the process above detailed, a fine 

 sphere of oil well suspended, and presenting, I will suppose, a diameter of six 

 to seven centimetres, we shall observe the following circumstances, which it is 

 important to notice before we proceed further : 



In the first place, the equilibrium, previously well established, is soon dis- 

 turbed of itself. At the end of a few minutes we see the sphere quit its place, 

 and rise with extreme slowness towards the upper part of the ambient liquid. 

 If a little alcohol be then added to restore the equilibrium, on treating the mix- 

 ture by the process of § 4, this equilibrium is again broken in the same manner 

 at the end of a certain time. In fine, it is only by continuing for some days to 

 maintain it by the successive addition of small quantities of alcohol that we 

 come to obtain a permanent equilibrium, wlii(?h is then no further disturbed, 

 except by an accidental cause, of which we shall speak in the following para- 

 graph. If the temperature does not foil below 18° centigr., the above phenomena 

 are the only ones observed ; but sometimes, if the temperature remains below 

 that limit, and always, if it is below 15°, another effect is manifested, namely, a 

 diminution in the transparency of the oil. 



These phenomena are owing to a gradual chemical action which takes 

 place between the oil and the alcoholic mixture. The first of these would be 

 very inconvenient in most of the experiments ; but, happily, it may be obvi- 

 ated. This can evidently be effected by employing the two liquids only when 

 they have already exerted upon one another all the action of which they are 

 capable. The oil and the alcoholic mixture which I used are now inert with 

 regard to one another, because, having been employed a great number of times, 

 they have had time to exercise the whole of their mutual action. Besides, it is 

 easy, in a short time, to bring the two liquids to that state of relative neu- 

 trality, by agitating them together in order to divide the oil, and thus to 

 accelerate the action, then separating them by a suitable process. This opera- 

 tion requires some precautious, which we shall examine in § 24, in order not to 

 interrupt the course of the memoir by details which are not now indispensable. 

 In all that follows we shall always suppose that two liquids thus prepared are 

 employed. 



7. Another cause disturbs the equilibrium between the sphere of oil and the 

 ambient liquid. This is the variations of temperature, which alter the equality 

 of the two densities ; and the degree of sensibility of such a system in this re- 

 spect would hardly be conceived. For example, when the vessel is carried 

 into a room a little warmer or colder than that in which it had been before, the 

 ' sphere soon falls in the first case and rises in the second. On the mere appli- 

 * cation of the hands to the outside of the vessel, it will be seen, after a few sec- 

 onds, that the sphere begins to fall. 



We must be continually on our guard against these effects of temperature ; 

 otherwise, they disturb the experiments. The following is a recent instance 



