What is potass?

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..."

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.

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."