I don't agree that this sounds anything like bowed cello notes, but it does sound a lot like an EBow. That's a device which generates feedback similar to what you can get by having the amp vibrate the guitar, but without the loudspeaker – instead, it directly vibrates the strings magnetically.
However, as per the article that Wikipedia and user1079505 linked to, Kim Thayil actually used amp feedback, not an EBow. That requires cranking the amp very loud (in particular, high gain) and standing close to it with the guitar. Even then it's quite difficult to control which notes / partials actually get sustained. So if you want to play this intro reliably, I would anyway recommend trying an EBow.
In addition to feedback, the guitar has a lot of distortion and a lot of reverb on it.