Chap. 5.] hollow Veficles. 483 



Spherical ^alls of different fizes may in this manner 

 be obferved proceeding from the, furface with more 

 or lefs rapidity. The more delicate rife fwiftly, and 

 foon traverfe the field of the lensj the larger fall back 

 into the cup, and, without mixing with the fluid, roll 

 upon its furface like a light powder, which obeys 

 every impulfe of the air. The lightnefs of thefe fmall 

 fpheres, their whitenefs, &c. give them an appearance 

 altogether different from folid globules j their perfect 

 refeinblance to the larger balls, that are feen floating 

 on the furface of the liquid, can leave no doubt of 

 their nature : it is fufficient to fee them to be con- 

 vinced that they are hollow bubbles, like thofe blown 

 from water a::d foap. M. Kratzenftein endeavoured 

 to eftimate their fize, by comparing them with a hair, 

 and /ound that they were twelve times fmaller than the 

 hair, the diameter of which was the three hundredth 

 part of -n inch, and confequently one of thefe was only 

 the three thoufand fix hundredth part of the fame 

 mealu re. 



Thefe bubbles may even be fometimes, it is faid, 

 obferved in a fog, or in a cloud, when the obferver is 

 ituated oa a hill. To this end M. de Sauffure ufed 

 a lens of one and an half or two inches focus, which 

 he held near his eye with one hand, in the other he 

 held any fmooth, flat, and polifhed furface, of a black 

 colour, as the bottom of a tortoife fhell box., bringing 

 this towards the lens till it was very near the focal* 

 diftance ; he then waited till the agitation of the air 

 brought fome pa'rticks of the cloud into the focus of 

 the lens; when the cloud was thick this focn hap- 

 pened, and he perceived round and white particles- 

 fome pafling with the rapidity of lightning, others 

 moving (lowly ; fome rolling on the furface of the 

 tortoife fhell, others ftriking againft it obliquely, and 

 I i 2 rebounding 



