are obscured, and bounded within the narrow limits of accidental obfervation.

The knowledge of the optical properties of colouring matters, is alfo effentially requifite to their due preparation and use. As the practice of dying, in its prefent ftate, is not regulated by any scientific rules, it is feldom improved by the introduction of new proceffes: and the methods of varying the ufes of the materals, which are already known, are rarely, afcertained without repeated trials.

All the operations of the art, excepting only a few which have arifen from accidental difcoveries, owe their origine to remote ages.

We learn from the teftimony of the facred writers, as well as of the later hiftorians, that the Indians, Egyptians, Phoenicians, and other ancient nations, excelled in the art of dying. From the accounts, which are delivered down to us, of the colouring materials which they used, and of the clothes which were dyed with them, we find evident proofs, that they were acquainted with the principles, as well as the practice, of the art.

The ancients did not attribute a reflective power to the colouring matter; but held, that the dyed clothes reflected lefs light, in proportion as they were more copiously stored with tinging particles. They estimated the richness and intenseness of the dye, by its approach to blackness. Pliny, who has recorded many curious circumftances relative to the arts, defcribes indigo, when

undiluted,

undiluted, as a black fubftance.* The fame author informs us, that the fpecies of Tyrian dye, which was most esteemed, was of a rose colour inclining to black; † and that the red was inferior to that which was blacker. He accurately diftinguishes the bright red colour, which is tranfmitted through the dyed clothes, from the dark hue, which they exhibit when viewed by incident light.

The inattention of later philofophers, and artists, to the reflective, and tranfmiffive, qualities of the conftituent parts of Coloured Substances, has, doubtless, impeded the progrefs and improvement

Plinii, L. XXXV. C. 6. Ab hoc maxima autoritas Indico. Ex India venit harundinum fpumæ adhærefcente limo. Cum teritur nigrum. At in diluendo mixturam purpuræ cæruleique mirabilem reddit.

+ Ib. L. IX. C. 36. Purpuræ florem illam tingendis veftibus expetitum, in mediis habent faucibus. Liquoris hic eft minimi in candida vena, unde pretiofus ille bibitur migrantis rofæ colore fublucens.

Ib. L. IX. C. 38. Rubens color nigrante deterior.

Ib. L. IX. C. 38. Laus ei fumma, color fanguinis concreti, nigricans afpe&u, idemque fufpectu refulgens.

It appears from the following paffage, cited from Macrobius, that the dealers in dyed clothes were accustomed to confider the colours, which they yielded by tranfmiffion, as well as those which they afforded, when viewed by incident light. "Cum de Tyria purpuræ, quam emi jufferat, obfcuritate quereretur (Auguftus) dicente venditore, erige altius et fufpice, his falibus ufus eft: Quid? ergo ut me populus Romanus dicat bene cultum, in folario ambulaturus fum ?" Macrob. Lib. 11. Saturn.

of

of the science of optics, and of the arts which are dependent upon it. This experimental research was undertaken, for the purpose of examining the optical qualities of fuch fubftances.

During the courfe of my inquiry, I have obferved, that the tranfition from physical experiments, to practical operations of dying, is eafy and obvious. For, the experiments, which I have made, with a view of investigating the origin and cause of colours, have guided me to the difcovery of several bright and permanent dyes, in the execution of which I have, principally, ufed cheap and common ingredients, that have not before been applied to fuch purposes.

I do not doubt, that a clear, and comprehensive, view of the principles of the art will open, to the artists who practise it, a fertile fource, from which, with certainty and facility, they may derive the discovery of new materials, and of the moft advantageous means of employing them.

The art of painting, alfo, will receive great advantage, from an accurate and precife conception of the principles, by means of which the colouring matters, endued with a reflective power, may be distinguished from others which tranfmit the rays, but do not reflect them.

The works of many painters greatly excel in the clearness and brightness of their colouring:* but,

Several of the greatest mafters practifed a method of colouring, which was, in fome refpects, conformable to

the

but, it is unquestionable that a fcientific knowledge of the nature of the colours, which they used, would have enabled even the greatest mafters to have communicated, to those works, a ftill higher degree of excellence.

I fhall not regret the labours which I have bestowed on the fubject of thefe pages, if they contribute to the advancement of those useful and elegant arts, which are of much importance to a commercial nation; or if, by the disclosure of phyfical truths, they extend the bounds of fcience, or open new paths to its improvement.

One of

the true principles of optics, and of their art. our most eminent painters has obferved, from a minute and accurate examination of fome of the most capital pictures of Titian, Tintoret, Paul Veronese, and other Italian, Flemish, and Dutch mafters, that they painted with transparent colours, upon a white ground. This practice was carried to the highest degree of perfection by Correggio. That great colourist grounded his pictures with black and white only and by the delicacy, clearlefs, and brilliancy of the tranfparent colours, which he laid on the grounds thus prepared, he gave to his compofitions a peculiar force and relief, and a near refemblance of nature. Few modern artists have been able to comprehend the means, by which those effects were produced. Leonardo Da Vinci, in his inftructive Treatife of Painting, recommends the ufe of white grounds, and transparent colours, as the true method of procuring the most brilliant colouring. Cap. 100. Sempre à quelli colori, che "vuoi che habibno bellezza, prepararai prima il campo "candidiffimo, e quefto dico de' colori che fono tranfparenti, perche a quelli che non fono trafparenti, non giova campo chiaro."

..

EXPERI

EXPERIMENTS and OBSERVATIONS on FERMENTS

and FERMENTATION; by which a MODE of exciting Fermentation in MALT LIQUORS, without the Aid of YEAST, is pointed out; with an ATTEMPT to form a new Theory of that Procefs. By THOMAS HENRY, F. R. S. Read April 20, 1785.

Nec manet ulla fui fimilis res; omnia migrant;
Omnia commutat Natura et vertere cogit.

LUCRETIUS.

F all the proceffes of chemistry, there is,

OF

perhaps, none, the phenomena of which have been lefs fatisfactorily explained, than those of Fermentation. The writers on Chemistry have been content to defcribe the feveral appearances, the progrefs and refult of fermentation, and have declined any inquiry into its primary causes, or into the mode by which the changes, induced by it, are effected in bodies, which are the objects of its action.

Within these few years, great changes have taken place in the theory of Chemistry. The

VOL. II.

S

important