He speaketh of the currency, not the coins in circulation.
The following applies to the silver coins of the Straits Settlements. As background, remember that since the Napoleonic wars, coins were standard coins or small change. The standard coins were supposed to be full weight. They could be freely minted (at a fee) and melted. Small change was token coin. It could at all times be exchanged for standard coins, but it was not full weight, could not be minted freely and was legal tender only up to a number of coins specified in the law.
From Victorian times to 1907, the 50 cent coin was the standard coin of the Straits. Silver coins had the same silver content (sc) and pro rata weight. In 1902, a dollar coin was added with the characteristics of the British trade dollar: somewhat less than the double weight and a higher sc as the 50 cents, so it was a second standard coin, but on a different standard. This made a silver arbitrage possible, as a dollar contained pro rata about 10% more silver than a 50 cents. In other words, if you would melt 9 dollars, you could produce about 20 coins of 50 cents. If mint fee and melting cost were low enough, dollars would have disappeared. Since the dollars are cheaper than the 50 cents, the arbitrage was presumably not workable in practice.
The reform of 1907 devalued the Straits dollar by 1/4th, the 50 cents was jacked up to the same sc but reduced in weight to half of the new dollar, so it remained a standard coin. The sc of the other silver coins was lowered, but their weight was unchanged.
For the sake of completeness, another reform was carried through after the first world war. The standard coins were lowered in weight by around 1/6th and silver content was lowered to the point where the coins would have started to look cheap after a bit of wear from circulation. Again, the weight of the small change remained the same.
The last reform was made in 1926. The sc was restored to the level of 1907 without changing weight. No standard coins were struck, possibly because the standard of 1907 had become impractical or because all coins had become small change.
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