VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 60S 



two men at Barton, about 3 miles off, saw between 3 and 4 o'clock that morn- 

 ing, in some shallow tubs, ice of the thickness of a crown-piece, and which was 

 not melted before 6. 



This unseasonable frost produced some remarkable effects. The aristae of the 

 barley, which was coming into ear, became brown and withered at their extre- 

 mities, as did the leaves of the oats; the rye had the appearance of being mil- 

 dewed; so that the farmers were alarmed for those crops. The wheat was not 

 much affected. The larch, Weymouth pine, and hardy Scotch fir, had the tips 

 of their leaves withered ; the first was particularly damaged, and made a shabby 

 appearance the rest of the summer. The leaves of some ashes, very much shel- 

 tered in my garden, suffered greatly. A walnut-tree received a second shock 

 (the first was from a severe frost on the 26th of May) which completed the ruin 

 of its crop. Cherry-trees, a standard peach-tree, filberd and hasel-nut-trees, 

 shed their leaves plentifully, and littered the walks as in autumn. The barberry- 

 bush was extremely pinched, as well as the hypericum perforatum and hirsutum: 

 as the last 2 are solstitial, and r .ther delicate plants, I wondered the less at their 

 sensibility ; but was much sun .ized to find that the vernal black-thorn and sweet 

 violet, the leaves of which o ,e would have thought must have acquired a perfect 

 firmness and strength, were .njured full as much. All these vegetables appeared 

 exactly as if a fire had been \ghted near them, that had shrivelled and discoloured 

 their leaves. penetr ibile frigus adurit. 



At the time this hav >ck was made among some of our hardy natives, the 

 exotic mulberry-tree was very little affected; a fig-tree, against a north-west wall, 

 remained unhurt, as well as the vine, on the other side, though just coming into 

 blossom. I speak of my own garden, which is high ; for in the low ones about 

 Bury, but a mile off, the fig-trees, in particular, were very much cut; and in 

 general all those gardens suffer more by frost than mine. 



XXX. On a Neiv Method of preparing a Test Liquor to show the Presence of 

 Acids and Alkalis in Chemical Mixtures. By Mr. James Watt. p. 41 9. 

 The syrup of violets was formerly the test of the point of saturation of mix- 

 tures of acids and alkalis, which was principally used; but since the late improve- 

 ments in chemistry it has been found not to be sufficiently accurate, and the in- 

 fusion of tournesol, or of an artificial preparation called litmus, have been sub- 

 stituted instead of it. The infusion of litmus is blue, and becomes red with acids. 

 It is sensible to the presence of 1 grain of common oil of vitriol, though it be 

 mixed with 1O0O0O grains of water; but as this infusion does not change its co- 

 lour on being mixed with alkaline liquors, in order to discover whether a liquor 

 be neutral or alkaline, it is necessary to add some vinegar to the litmus, so as 

 just to turn the infusion red, which will then be restored to its blue colour, by 



