197 
A phenomenon, not less interesting and astonishing, is the ap- 
pearance of floating islands on this lake. These rise, at uncertain 
periods, from the bottom of the lake : sometimes one, and some- 
times two, have arisen in a year ; and, at other times, several years 
may elapse without their appearance. Their time of remaining at 
the surface is also very uncertain ; some sinking again within twenty- 
four hours ; whilst others remain for six weeks, two months, or even 
longer, before they return to the bottom of the lake ; which they in 
general do without having suflered any change in their form : but 
sometimes they burst, and are so widely rent, that boats can pass 
between the separated parts. One rose in 1798, which was one 
hundred and eighty yards long, and fifty yards wide. They some- 
times show above a foot high of land, out of the water ; and some 
have been measured which have been seven yards in thickness. The 
upper part of these masses, for about two feet in depth, is common 
mud ; beneath this it becomes less compact, and the remaining part 
is found to be formed of decayed leaves, and roots of plants and trees. 
A pole being passed three or four feet into them, a considerable 
quantity of air rushes out on its being withdrawn : this air smells 
like gunpowder ; and it is said that, if collected in a bottle, it will 
take fire if exposed to the flame of a candle*. 
The most prominent circumstances in the foregoing relation are, 
the matter of which these islands are chiefly composed ; the gaseous 
matter which is so plentifully liberated, from an opening made into 
its substance by means of a pole; and the extraordinary impulse 
and agitation by which the water of the lake is affected. These 
several circumstances we will therefore subject to a careful and im- 
partial examination. 
All, except about two feet of the upper part, of these islands, 
appears to have been formed of the decayed leaves and roots of 
# Phil. Mag. No. XLII. 
