What is potass?

The Oxford English Dictionary has potass from potash:

An alkaline substance obtained originally by lixiviating or leaching the ashes of terrestrial vegetables and evaporating the solution in large iron pans or pots (whence the name). Chemically, this is a crude form of potassium carbonate (more or less mixed with sulphate, chloride, and empyreumatic substances), but was long thought to be (when freed from impurities) a simple substance.

And further defines it as:

Used also to include the impure carbonate of soda, barilla. [Obscure]

barilla is defined as:

An impure alkali produced by burning the dried [maritime plant barilla (Salsola Soda) which grows extensively in Spain, Sicily, and the Canary Islands] and allied species; formerly imported in large quantities, and used in the manufacture of soda [...]

Emphasis mine.

So it's basically an old-time crude form of soda.

So he's having a (probably, by modern standards, quite disgusting and strong) whisky and soda.

The point the author might be making is it's not a lady's drink. But something a real man would drink to "refresh" himself. Perhaps with some intended irony.

As indicated by @RobSter, Potass/Potass water/Kali potass was an sparkling acid beverage composed of water and nitrate salts, supposed to have medicinal virtues.

Source: The new London dispensatory, containing a treaty of the Pharmacopœia by Thomas Cox - 1824.

The spelling of "whisky" sent me looking toward Britain. From the Aesclepiad Advertiser, 1891, comes this advert from a firm of beverage bottlers in Croydon:

I suppose one could mix it with whisky. It seems to be a brutal thing to do to good whiskey, though.

It's said that a certain Kentucky gentleman was asked why he always closed his eyes when drinking a mint julep. "Suh," he replied, "the sight of good liquor makes mah mouth water, and ah don't intend fo' mah drink to be diluted..."

I believe, due to an affection for British Victorian-era fiction, that this is a "period" allusion to mixing a drink by using an old-fashioned "soda siphon;" not the current style of seltzer-bottle that is recharged using pressurized cylinders of carbon dioxide. Potassium bicarbonate---"potass"---and an acetic agent combined with water in the wire-reinforced "carboy" or bottle and provided the "fizz."

The chap had a "whisky and soda," in other words. (Cheers!)

At a time when taking one's liquor at room temperature was normal, putting a bit of fizz (and, probably, a slight "tang" in the water, from the acetic agent used to generate the bubbles of gas) in one's liquor was the "sophisticated" way to have a "mixed drink."